[autoscaler] Document gcp subnet config (#3783)

Adds info to the gcp example yaml on using shared subnets.

python/ray/autoscaler/gcp/example-full.yaml

@@ -66,6 +66,15 @@ head_node:
     # Additional options can be found in in the compute docs at
     # https://cloud.google.com/compute/docs/reference/rest/v1/instances/insert
 
+    # If the network interface is specified as below in both head and worker 
+    # nodes, the manual network config is used.  Otherwise an existing subnet is
+    # used.  To use a shared subnet, ask the subnet owner to grant permission 
+    # for 'compute.subnetworks.use' to the ray autoscaler account...
+    # networkInterfaces:
+    #   - kind: compute#networkInterface
+    #     subnetwork: path/to/subnet
+    #     aliasIpRanges: []
+
 worker_nodes:
     machineType: n1-standard-2
     disks: