Update upgrader.md ... safety first

doc/update/upgrader.md

@@ -28,3 +28,15 @@ __GitLab Upgrader is available only for GitLab version 6.4.2 or higher__
 
     sudo service gitlab start
     sudo service nginx restart
+
+### 4. Check application status
+
+Check if GitLab and its environment are configured correctly:
+
+    sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production
+    
+To make sure you didn't miss anything run a more thorough check with:
+
+    sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production
+    
+If all items are green, then congratulations upgrade is complete!