Ask for CHANGELOG entries in Merge Requests

CONTRIBUTING.md

 These are the contributing guidelines for omnibus-gitlab issues and merge
 requests.
+## General guidelines
+
 - Include the omnibus-gitlab version when discussing behavior: `dpkg-query -W
   gitlab` or `rpm -q gitlab`.
 - Be careful when pasting log output of `gitlab-ctl reconfigure`; Chef happily
   writes secrets to the log.
+
+## Merge request guidelines
+
+- Please add a CHANGELOG entry for your contribution