Add docs for labels

Fix kubernetes#441

Fix kubernetes#241

Issue kubernetes#311

docs/labels.md

@@ -0,0 +1,29 @@
+# Labels
+
+There are two main types of labels that kops can create:
+
+* `CloudLabels` become tags in AWS on the instances
+* `NodeLabels` become labels on the k8s Node objects
+
+Both are specified at the InstanceGroup level.
+
+A nice use for CloudLabels is to specify [AWS cost allocation tags](http://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/cost-alloc-tags.html)
+
+An example:
+
+`kops edit ig nodes`
+
+```
+...
+  nodeLabels:
+    spot: "false"
+  cloudLabels:
+    team: me
+    project: ion
+...
+```
+
+Note that keys and values are strings, so you need quotes around values that YAML
+ would otherwise treat as numbers or booleans.
+
+To apply changes, you'll need to do a `kops update cluster` and then likely a `kops rolling-update cluster`