Add author and issue id warning.

CHANGELOG

 v 6.1.0
+  - Project specific IDs for issues, mr, milestones
+    Above items will get a new id and for example all bookmarked issue urls will change.
+    Old issue urls are redirected to the new one if the issue id is too high for an internal id.
   - Description field added to Merge Request
   - API: Sudo api calls (Izaak Alpert)
   - API: Group membership api (Izaak Alpert)
   - Improved commit diff
   - Improved large commit handling (Boyan Tabakov)
   - Rewrite: Init script now less prone to errors and keeps better track of the service.
-  - Link issues, merge requests, and commits when they reference each other with GFM
+  - Link issues, merge requests, and commits when they reference each other with GFM (Ash Wilson)
   - Close issues automatically when pushing commits with a special message
-  - Project internal ids for issues, mr, milestones
   - Improve user removal from admin area
   - Invalidate events cache when project was moved
   - Remove deprecated classes and rake tasks