Add a tip on how to test regular expressions

config/gitlab.yml.example

@@ -67,6 +67,7 @@ production: &base
     # If a commit message matches this regular expression, all issues referenced from the matched text will be closed.
     # This happens when the commit is pushed or merged into the default branch of a project.
     # When not specified the default issue_closing_pattern as specified below will be used.
+    # Tip: you can test your closing pattern at http://rubular.com
     # issue_closing_pattern: '([Cc]lose[sd]|[Ff]ixe[sd]) #(\d+)'
 
     ## Default project features settings