add instructions for setting up RBAC roles required on GKE

[rade]

Our k8s installation instructions do not mention the magic GKE incantation

kubectl create clusterrolebinding "cluster-admin-$(whoami)" --clusterrole=cluster-admin --user="$(gcloud config get-value core/account)"

that is required for making the installation not explode with something like

Error from server (Forbidden): clusterroles.rbac.authorization.k8s.io "weave-scope" is forbidden: attempt to grant extra privileges: [PolicyRule{Resources:["*"], APIGroups:["*"], Verbs:["*"]} PolicyRule{NonResourceURLs:["*"], Verbs:["*"]}] user=&{allan@17.media  [system:authenticated] map[]} ownerrules=[PolicyRule{Resources:["selfsubjectaccessreviews"], APIGroups:["authorization.k8s.io"], Verbs:["create"]} PolicyRule{NonResourceURLs:["/api" "/api/*" "/apis" "/apis/*" "/healthz" "/swaggerapi" "/swaggerapi/*" "/version"], Verbs:["get"]} PolicyRule{NonResourceURLs:["/swagger-2.0.0.pb-v1"], Verbs:["get"]} PolicyRule{NonResourceURLs:["/swagger.json"], Verbs:["get"]}] ruleResolutionErrors=[]