Add CHANGELOG item

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

CHANGELOG

@@ -16,6 +16,7 @@ v 7.13.0 (unreleased)
   - API request /projects/:project_id/merge_requests?state=closed will return only closed merge requests without merged one. If you need ones that were merged - use state=merged.  
   - Allow Administrators to filter the user list by those with or without Two-factor Authentication enabled.
   - Show a user's Two-factor Authentication status in the administration area.
+  - Improve performance for issue and merge request pages 
 
 v 7.12.0 (unreleased)
   - Fix Error 500 when one user attempts to access a personal, internal snippet (Stan Hu)