Merge branch 'docs-add-existing-k8s-integration-info' into 'master'

Added info on getting k8s integration for existing cluster

See merge request gitlab-org/gitlab-ce!21783

doc/user/project/clusters/index.md

@@ -113,6 +113,14 @@ To add an existing Kubernetes cluster to your project:
 After a couple of minutes, your cluster will be ready to go. You can now proceed
 to install some [pre-defined applications](#installing-applications).
 
+To determine the:
+
+- API URL, run `kubectl cluster-info | grep 'Kubernetes master' | awk '/http/ {print $NF}'`.
+- Token:
+  1. List the secrets by running: `kubectl get secrets`. Note the name of the secret you need the token for.
+  1. Get the token for the appropriate secret by running: `kubectl get secret <SECRET_NAME> -o jsonpath="{['data']['token']}" | base64 -D`.
+- CA certificate, run `kubectl get secret <secret name> -o jsonpath="{['data']['ca\.crt']}" | base64 -D`.
+
 ## Security implications
 
 CAUTION: **Important:**