Add --listening-url-file to cockroach start docs

Also document passing --port=0 to have the node's
port automatically assigned.

Fixes #1350

start-a-node.md

@@ -39,11 +39,12 @@ Flag | Description
 `--http-port` | The port to bind to for Admin UI HTTP requests. <br><br>**Default:** `8080`
 `--insecure` | Run in insecure mode. If this flag is not set, the `--certs-dir` flag must point to valid certificates.<br><br>**Default:** `false`
 `--join`<br>`-j` | The address for connecting the node to an existing cluster. When starting the first node, leave this flag out. When starting subsequent nodes, set this flag to the address of any existing node.<br><br>Optionally, you can specify the addresses of multiple existing nodes as a comma-separated list, using multiple `--join` flags, or using a combination of these approaches, for example: <br><br>`--join=localhost:1234,localhost:2345`<br>`--join=localhost:1234 --join=localhost:2345`<br>`--join=localhost:1234,localhost:2345 --join=localhost:3456`
+`--listening-url-file` | The file to which the node's SQL connection URL will be written on successful startup, in addition to being printed to [standard output](#standard-output).<br><br>This is particularly helpful in identifying the node's port when the port is assigned automatically (`--port=0`).
 `--locality` | Arbitrary key-value pairs that describe the locality of the node. Locality might include country, region, datacenter, rack, etc.<br><br>CockroachDB attempts to spread replicas evenly across the cluster based on locality, with the order determining the priority. The keys themselves and the order of key-value pairs must be the same on all nodes, for example:<br><br>`--locality=region=east,datacenter=us-east-1`<br>`--locality=region=west,datacenter=us-west-1`<br><br>These can be used to influence the location of data replicas. See [Configure Replication Zones](configure-replication-zones.html#replication-constraints) for full details.
 `--max-offset` | The maximum allowed clock offset for the cluster. If observed clock offsets exceed this limit, servers will crash to minimize the likelihood of reading inconsistent data. Increasing this value will increase the time to recovery of failures as well as the frequency of uncertainty-based read restarts.<br><br>Note that this value must be the same on all nodes in the cluster. In order to change it, every node in the cluster must be stopped and restarted with the new value.<br><br>**Default:** 500ms
 `--max-sql-memory` | The total size for storage of temporary data for SQL clients, including prepared queries and intermediate data rows during query execution. This can be in any bytes-based unit, for example:<br><br>`--max-sql-memory=10000000000 ----> 1000000000 bytes`<br>`--max-sql-memory=1GB ----> 1000000000 bytes`<br>`--max-sql-memory=1GiB ----> 1073741824 bytes`<br><br>**Default:** 25% of total system memory (excluding swap), or 512MiB if the memory size cannot be determined
 `--pid-file` | The file to which the node's process ID will be written on successful startup. When this flag is not set, the process ID is not written to file.
-`--port`<br>`-p` | The port to bind to for internal and client communication. <br><br>**Env Variable:** `COCKROACH_PORT`<br>**Default:** `26257`
+`--port`<br>`-p` | The port to bind to for internal and client communication.<br><br>To have an unused port automatically assigned to the node, pass `--port=0`.<br><br>**Env Variable:** `COCKROACH_PORT`<br>**Default:** `26257`
 `--store`<br>`-s` | The file path to a storage device and, optionally, store attributes and maximum size. When using multiple storage devices for a node, this flag must be specified separately for each device, for example: <br><br>`--store=/mnt/ssd01 --store=/mnt/ssd02` <br><br>For more details, see [`store`](#store) below.
 
 #### `store`