Mention protected branch force push

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

CHANGELOG

@@ -7,6 +7,7 @@ v 6.8.0
   - Make the repository downloads path configurable
   - Create branches via API (sponsored by O'Reilly Media)
   - Changed permission of gitlab-satellites directory not to be world accessible
+  - Protected branch does not allow force push
 
 v 6.7.3
   - Fix the merge notification email not being sent (Pierre de La Morinerie)

app/views/projects/protected_branches/index.html.haml

@@ -9,6 +9,7 @@
       %ul
         %li keep stable branches secured
         %li forced code review before merge to protected branches
+        %li prevents branch from force push
       %p Read more about project permissions #{link_to "here", help_permissions_path, class: "underlined-link"}
 
     - if can? current_user, :admin_project, @project