Add docs for upgrader script

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

doc/update/upgrader.md

+# 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.
+
+### 0. Backup
+
+    cd /home/git/gitlab
+    sudo -u git -H bundle exec rake gitlab:backup:create RAILS_ENV=production
+
+### 1. Stop server
+
+    sudo service gitlab stop
+
+### 2. Run gitlab upgrade tool
+
+    cd /home/git/gitlab
+    sudo -u git -H ruby script/upgrade.rb
+
+    # it also supports -y option to avouid user input
+    # sudo -u git -H ruby script/upgrade.rb -y
+   
+
+### 3. Start application
+
+    sudo service gitlab start
+    sudo service nginx restart