Add new entries to CHANGELOG

CHANGELOG

 v 6.9.0
   - Store Rails cache data in the Redis `cache:gitlab` namespace
   - Adjust MySQL limits for existing installations
-  - Add db index on project_id+iid column. This prevents duplicate on iid.
+  - Add db index on project_id+iid column. This prevents duplicate on iid (During migration duplicates will be removed)
+  - Markdown preview or diff during editing via web editor (Evgeniy Sokovikov)
+  - Give the Rails cache its own Redis namespace
+  - Add ability to set different ssh host, if different from http/https
+  - Fix syntax highlighting for code comments blocks
+  - Improve comments loading logic
 
 v 6.8.0
   - Ability to at mention users that are participating in issue and merge req. discussion