Add important note to move custom Git hooks

doc/administration/gitaly/index.md

@@ -328,6 +328,8 @@ When you tail the Gitaly logs on your Gitaly server you should see requests
 coming in. One sure way to trigger a Gitaly request is to clone a repository
 from your GitLab server over HTTP.
 
+DANGER: **Danger:** If you have [custom server-side Git hooks](../blob/master/doc/administration/custom_hooks.md#custom-server-side-git-hooks) configured, either per repository or globally, you must move these to the Gitaly node. If you have multiple Gitaly nodes, copy your custom hook(s) to all nodes.
+
 ### Disabling the Gitaly service in a cluster environment
 
 If you are running Gitaly [as a remote