Added CHANGELOG for housekeeping and changed GITLAB_SHELL_VERSION to 2.6.10

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.4.0 (unreleased)
+  - Add housekeeping function to project settings page
   - Expire view caches when application settings change (e.g. Gravatar disabled) (Stan Hu)
   - Don't notify users twice if they are both project watchers and subscribers (Stan Hu)
   - Implement new UI for group page

GITLAB_SHELL_VERSION

-2.6.9
+2.6.10