Add entry to changelog.

CHANGELOG

@@ -8,6 +8,7 @@ v 7.0.0
   - Move protected branches page to Project settings area
   - Redirect to Files view when create new branch via UI
   - Drag and drop upload of image in every markdown-area (Earle Randolph Bunao and Neil Francis Calabroso)
+  - Refactor the markdown relative links processing
 
 v 6.9.0
   - Store Rails cache data in the Redis `cache:gitlab` namespace