Add Notification secret into gitlab.yml.template

config/gitlab.yml.example

@@ -431,6 +431,7 @@ production: &base
     # key: config/registry.key
     # path: shared/registry
     # issuer: gitlab-issuer
+    # notification_secret: '' # only set it when you use Geo replication feature without built-in Registry
 
     # Add notification settings if you plan to use Geo Replication for the registry
     # notifications: