Add docs about requirement for basic auth and cert

Document that GitLab need the cluster to be created with basic auth and
cert and why. Also note we have started explicitly enabling this so that
we are ready when these settings are disabled by default in GKE 1.12

doc/user/project/clusters/index.md

@@ -75,6 +75,14 @@ new 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).
 
+NOTE: **Note:**
+GitLab requires basic authentication enabled and a client certificate issued for
+the cluster in order to setup an [initial service
+account](#access-controls). Starting from [GitLab
+11.10](https://gitlab.com/gitlab-org/gitlab-ce/issues/58208), the cluster
+creation process will explicitly request that basic authentication and
+client certificate is enabled.
+
 ## Adding an existing Kubernetes cluster
 
 To add an existing Kubernetes cluster to your project: