Add warning for issue visibility.

Author: dosire

CHANGELOG

@@ -1,5 +1,6 @@
 v 6.2.0
-  - Public projects are visible from the outside
+  - Public project pages are now visible to everyone (files, issues, wik, etc.)
+    THIS MEANS YOUR ISSUES AND WIKI FOR PUBLIC PROJECTS ARE PUBLICLY VISIBLE AFTER THE UPGRADE
   - Add group access to permissions page
   - Require current password to change one
   - Group owner or admin can remove other group owners