update upgrade guide with backup, db specific commands, config changes, ...

Created by: bbodenmiller

...check, and restore

Closes #4047

doc/update/5.1-to-5.2.md

 # From 5.1 to 5.2
+### 0. Backup
+
+It's useful to make a backup just in case things go south:
+(With MySQL, this may require granting "LOCK TABLES" privileges to the GitLab user on the database version)
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H RAILS_ENV=production bundle exec rake gitlab:backup:create
+```
+
 ### 1. Stop server
     sudo service gitlab stop
@@ -20,14 +30,59 @@ sudo -u git -H git fetch
 sudo -u git -H git checkout v1.4.0
 ```
-### 4. Install libs, migrations etc
+### 4. Install libs, migrations, etc.
 ```bash
 cd /home/git/gitlab
+
+# MySQL
 sudo -u git -H bundle install --without development test postgres --deployment
+
+#PostgreSQL
+sudo -u git -H bundle install --without development test mysql --deployment
+
 sudo -u git -H bundle exec rake db:migrate RAILS_ENV=production
 ```
-### 5. Start application
+### 5. Update config files
+
+* Make `/home/git/gitlab/config/gitlab.yml` same as https://github.com/gitlabhq/gitlabhq/blob/5-2-stable/config/gitlab.yml.example but with your settings.
+* Make `/home/git/gitlab/config/puma.rb` same as https://github.com/gitlabhq/gitlabhq/blob/5-2-stable/config/puma.rb.example but with your settings.
+
+### 6. Update Init script
+
+```bash
+sudo rm /etc/init.d/gitlab
+sudo curl --output /etc/init.d/gitlab https://raw.github.com/gitlabhq/gitlabhq/5-2-stable/lib/support/init.d/gitlab
+sudo chmod +x /etc/init.d/gitlab
+```
+
+### 6. Start application
     sudo service gitlab start
+    sudo service nginx restart
+
+### 7. 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 complete!
+
+## Things went south? Revert to previous version (5.1)
+
+### 1. Revert the code to the previous version
+Follow the [`upgrade guide from 5.0 to 5.1`](5.0-to-5.1.md), except for the database migration 
+(The backup is already migrated to the previous version)
+
+### 2. Restore from the backup:
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H RAILS_ENV=production bundle exec rake gitlab:backup:restore
+```