Add CHANGELOG entry [ci skip]

CHANGELOG.md

@@ -5,6 +5,8 @@ Please view this file on the master branch, on stable branches it's out of date.
   - Fix HipChat notifications rendering (airatshigapov, eisnerd)
   - Simpler arguments passed to named_route on toggle_award_url helper method
+  - Fix discussion thread from emails for merge requests. !7010
+
 ## 8.13.0 (2016-10-22)
   - Fix save button on project pipeline settings page. (!6955)