docs: Adds documentation for traffic splitting support in kn

 - Add summary about operations, their usage and examples
 - Flags precendence explained
 - Summary column explaining if repetition is allowed or not
 - Example of different scenarios
 - Add case where tagOfOneTarget==revisionOfAnotherTarget, prefrence goes to tagOfOneTarget
   as Tags are unique while revions in a traffic block aren't.
 - Updating tag requires using `--untag` flag
 - Update doc to mention the tag update operation and example
 - Add notes about
	- assigning multiple tags for same revision
        - creating a new target in traffic if same revision is tagged with new tag
 - Update CHANGELOG

CHANGELOG.adoc

@@ -42,6 +42,10 @@
 | Add --revision-name flag
 | https://github.com/knative/client/pull/282[#282]
 
+| 🎁
+| Add documentation for traffic splitting and tagging targets
+| https://github.com/knative/client/pull/331[#331]
+
 |===
 
 ## v0.2.0 (2019-07-10)

docs/TrafficSplitting.md

@@ -0,0 +1,166 @@
+# Traffic Splitting
+
+kn can help you control which revisions get traffic in your Knative Service.
+
+A Knative Service has a traffic mapping, which is a mapping of revisions of the Service to percent amounts of traffic, along with the ability to create unique URLs for particular revisions. It also has the ability to assign traffic to the latest Revision, which can change as new revisions are created.
+
+## Operations
+
+`kn` supports the following traffic operations on traffic block of a Service as part of `kn service update` command.
+
+### 1. Split Traffic:
+
+Every time you update the Service's template, a new Revision is created with 100% traffic routing to it. With every update to the Configuration of the
+Service, a new Revision is created with Service route pointing all the traffic to the latest ready Revision by default.
+However, we can change this behavior by defining which Revision gets what portion of the traffic.
+For example, you might want to carefully roll out 10% of traffic to your new Revision before putting all traffic on it.
+
+Updating the traffic is performed by `kn service update` with the `--traffic` flag.
+
+- **`--traffic RevisionName=Percent`**
+  - `--traffic` flag requires two values separated by a `=`
+  - `RevisionName` string refers to name of Revision and `Percent` integer denotes the traffic portion to assign for this Revision
+  - `Percent` integer denotes the traffic portion to assign to the this Revision
+  - Use identifier `@latest` for RevisionName to refer to the latest ready Revision of the service, you can use this identifier only once with `--traffic` flag
+  - If the `service update` command is updating the Configuration values for the Service (along with traffic flags), `@latest` reference will be pointed to the created Revision after applying the update
+  - `--traffic` flag can be specified multiple times and is valid only if the sum of `Percent` values in all flags add up to 100
+
+### 2. Tag Revisions:
+A tag in traffic block of Service creates a custom URL, which points to the referenced Revision.
+A user can define a unique tag for an available Revision of the Service thereby generating a custom URL of format `http(s)://TAG-SERVICE.DOMAIN`.
+A given tag must be unique in traffic block of the Service. `kn` supports assigning and unassigning custom tags for revisions of the Service as part of `kn service update` command as follow:
+
+- **`--tag RevisionName=Tag`**
+  - `--tag` flag requires two values separated by a `=`
+  - `RevisionName` string refers to name of the `Revision`
+  - `Tag` string denotes the custom tag to be given for this Revision
+  - Use identifier `@latest` for RevisionName to refer to the latest ready Revision of the service, you can use this identifier only once with `--tag` flag
+  - If the `service update` command is updating the Configuration values for the Service (along with tag flags), `@latest` reference will be pointed to the created Revision after applying the update
+  - `--tag` flag can be specified multiple times
+  - `--tag` flag may assign different tags to same Revision, repeat the flag with same Revision name and different tags
+
+*Note*:
+- If you have assigned a tag to a particular Revision, a user can reference the Revision by its tag in `--traffic` flag as `--traffic Tag=Percent`.
+- User can specify `--traffic` and `--tag` flags in same `service update` command and allowed to use the new tag for Revision reference
+  for eg: `kn service update svc --tag echo-abcde=stable --traffic stable=100`.
+
+### 3. Untag Revisions:
+Assigned tags to revisions in traffic block can be unassigned, thereby removing any custom URLs. A user can unassign the tags for revisions as part of `kn service update` command as follow:
+
+- **`--untag Tag`**
+  - `--untag` flag requires one value
+  - `tag` string denotes the unique tag in traffic block of the Service which needs to be unassigned and thereby also removing respective custom URL
+  - `--untag` flag can be specified multiple times
+
+*Note*:
+- If a Revision is untagged and it is assigned 0% of the traffic, it is removed from the traffic block entirely.
+
+## Flag operation precedence
+- All the traffic-related flags can bespecified together in a single `kn service update` command, thus `kn` defines the precedence of these flags in which operations are evaluated.
+- The order of the flags specified in the command are **not** taken into account.
+- The precedence of flag(s) as they are evaluated by `kn`:
+
+1. `--untag`: First, all the referenced tags by this flag are removed from the traffic block
+2. `--tag`: Second, all the revisions are tagged as specified in the traffic block
+3. `--traffic`: Third, all the referenced revisions (or tags) are assigned given portion of traffic split
+
+*Note*:
+- Use comma separated value pairs or separate value pairs:
+  For example: `--tag echo-v1=v1,echo-v2=v2` is same as `--tag echo-v1=v1 --tag echo-v2=v2`
+  similarly, `--traffic v1=50,v2=50` is same as `--traffic v1=50 --traffic v2=50`
+  also, `--untag v1,v2` is same `--untag v1 --untag v2`
+- `--untag` flag's action takes first precedence. If `--untag` and `--tag` flags specified together for revisions, all the mentioned tags will be unassigned from revisions first, followed by tagging them.
+- Updating a tag for Revision requires untagging its existing tag and assigning a new tag.
+  For example, to update tag from 'testing' to 'staging' for Revision 'echo-v1': `--untag testing --tag staging`
+- If a tag is assigned to a Revision and user tries to use same tag for another Revision, an error is raised.
+- If a tag is assigned to a Revision and user tries to tag the same Revision with a new tag again, a new target is created with 0% traffic and new tag.
+- To assign multiple tags to same Revision, do `--tag sameRevisionName=tagName1 --tag sameRevisionName=tagName2`.
+- Repetition of `@latest` identifier for `--traffic` and `--tag` flags is not allowed.
+- Repetition of revisionName or tagName for `--traffic` flag is not allowed;
+  - If you want to have same revision present in multiple targets, use different tags to uniquely identify targets,
+    and use unique tag names to assign traffic portions.
+    For example: `kn service update svc --tag echo-v1=v1,echo-v2=v2, --traffic v1=50,v2=50`
+- If one traffic block target has revisionName which matches tagName of another target, tagName is looked up first for assigning traffic.
+  For example, `--traffic targetRef:percent` will assign mentioned `percent` to target where tag is `targetRef`.
+  **Tags are unique** in traffic block, **RevisionNames are not**, use tag reference in such scenarios.
+
+
+## Summary
+Following table shows quick summary for traffic splitting flags, their value formats, operation to perform and *Repetition* column denotes if repeating the particular value of flag is allowed in a `kn service update` command.
+
+
+Flag | Value(s) | Operation | Repetition
+:--- | :---: | :--- | :---:
+`--traffic` | `RevisionName=Percent` | Give `Percent` traffic to `RevisionName` | :heavy_check_mark:
+`--traffic` | `Tag=Percent` | Give `Percent` traffic to the Revision having `Tag` | :heavy_check_mark:
+`--traffic` | `@latest=Percent` | Give `Percent` traffic to the latest ready Revision | :heavy_multiplication_x:
+`--tag` | `RevisionName=Tag` | Give `Tag` to `RevisionName` | :heavy_check_mark:
+`--tag` | `@latest=Tag` | Give `Tag` to the latest ready Revision | :heavy_multiplication_x:
+`--untag` | `Tag` | Remove `Tag` from Revision | :heavy_check_mark:
+
+
+## Examples
+1. Tag revisions `echo-v1` and `echo-v2` as `stable` and `staging` respectively:
+```
+kn service update svc --tag echo-v1=stable --tag echo-v2=staging
+```
+
+2. Ramp up/down revision `echo-v3` to `20%`, adjusting other traffic to accommodate:
+```
+kn service update svc --traffic echo-v3=20 --traffic echo-v2=80
+```
+
+3. Give revision `echo-v3` tag `candidate`, without otherwise changing any traffic split:
+```
+kn service update svc --tag echo-v3=candidate
+```
+
+4. Give `echo-v3` tag `candidate`, and `2%` of traffic adjusting other traffic to go to revision `echo-v2`:
+```
+kn service update svc --tag echo-v3=candidate --traffic candidate=2 --traffic echo-v2=98
+```
+
+6. Update tag for `echo-v3` from `candidate` to `current`:
+```
+kn service update svc --untag candidate --tag echo-v3=current
+```
+
+7. Remove tag `current` from `echo-v3`:
+```
+kn service update svc --untag current
+```
+
+8. Remove `echo-v3` having no tag(s) entirely, adjusting `echo-v2` to fill up:
+```
+kn service update svc --traffic echo-v2=100    # a target having no-tags or no-traffic gets removed
+```
+
+9. Remove `echo-v1` and its tag `old` from the traffic assignments entirely:
+```
+kn service update svc --untag old --traffic echo-v1=0
+```
+
+10. Tag revision `echo-v1` with `stable` as well as `current`, and `50-50%` traffic split to each:
+```
+kn service update svc --tag echo-v1=stable,echo-v2=current --traffic stable=50,current=50
+```
+
+11. Revert all the traffic to the latest ready revision of service:
+```
+kn service update svc --traffic @latest=100
+```
+
+12. Tag latest ready revision of service as `current`:
+```
+kn service update svc --tag @latest=current
+```
+
+13. Update tag for `echo-v4` to `testing` and assign all traffic to it:
+```
+kn service update svc --untag oldv4 --tag echo-v4=testing --traffic testing=100
+```
+
+14. Update `latest` tag of `echo-v1` with `old` tag, give `latest` to `echo-v2`:
+```
+kn service update svc --untag latest --tag echo-v1=old --tag echo-v2=latest
+```