Add note about client secret

Author: pjlast

docs/admin/code_hosts/github.mdx

@@ -143,6 +143,8 @@ To add an existing GitHub App:
 4. Click **Add GitHub App** to save the configuration.
 5. Once added, you can manage installations and create code host connections just like with apps created through Sourcegraph.
 
+> NOTE: You'll still need to create a client secret on GitHub and [set up an auth provider](/admin/auth#github) if you would like to enable repository permissions.
+
 When creating code host connections for your existing GitHub App, you can choose to sync repositories from a specific installation or from all installations by omitting the `installationID` field (see [Syncing repositories from all installations](#syncing-repositories-from-all-installations)).
 
 > NOTE: For GitHub Enterprise Apps, this method allows you to use a single app across multiple organizations within your enterprise, avoiding the need to create separate public apps or multiple private apps for each organization.