Document that GKE requires current user to have cluster-admin role to create new roles

[TimMoore]

The documentation for Role-Based Access Control in Kubernetes Service Discovery informs users to create a role and role binding to allow service pods to access the Kubernetes API resources needed for service discovery.

Google Cloud users have informed us that these instructions don't work unless you have explicitly granted your own user permission to create roles.

See:

• Resolve attempt failed! Cause: spray.json.DeserializationException: Object is missing required member 'items' lagom/online-auction-scala#78 (comment)
• https://github.com/jcbsmpsn/gke-rbac-walkthrough/blob/master/create-roles.md
• https://cloud.google.com/kubernetes-engine/docs/how-to/role-based-access-control#prerequisites_for_using_role-based_access_control

These examples show binding the cluster-admin ClusterRole to the current user:

kubectl create clusterrolebinding cluster-admin-binding \
--clusterrole cluster-admin --user $(gcloud config get-value account)

It's probably enough to link to Google's guide with a note.