Added CHANGELOG message for fixing MR webhook

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 7.13.0 (unreleased)
+  - Fix Merge Request webhook to properly fire "merge" action when accepted from the web UI
   - Fix invalid timestamps in RSS feeds (Rowan Wookey)
   - Fix error when deleting a user who has projects (Stan Hu)
   - Fix downloading of patches on public merge requests when user logged out (Stan Hu)