GitLab Upgrader requires v6.4.0+

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

doc/update/upgrader.md

-# GitLab upgrader 
+# GitLab Upgrader 
 
 GitLab Upgrader - ruby script that allows you easily upgrade GitLab to latest minor version.
 Ex. it can update your application from 6.4 to latest GitLab 6 version (like 6.5.1).
 You still need to create backup and manually restart GitLab but all other operations can be done by upgrade tool.
 
+__GitLab Upgrader is available only for version 6.4 or higher__
+
 ### 0. Backup
 
     cd /home/git/gitlab