Add changelog entry

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

CHANGELOG

+v 6.7.0
+  - Add edit file button to MergeRequest diff
+
 v 6.6.0
   - Retrieving user ssh keys publically(github style): http://__HOST__/__USERNAME__.keys
   - Permissions: Developer now can manage issue tracker (modify any issue)