Add link to Google billing account

doc/user/project/clusters/index.md

@@ -24,7 +24,9 @@ prerequisites must be met:
   be enabled in GitLab at the instance level. If that's not the case, ask your
   administrator to enable it.
 - Your associated Google account must have the right privileges to manage
-  clusters on GKE.
+  clusters on GKE. That would mean that a
+  [billing account](https://cloud.google.com/billing/docs/how-to/manage-billing-account)
+  must be set up.
 - You must have Master [permissions] in order to be able to access the **Cluster**
   page.