Add missing update docs from 5.2 to 5.3 to 5.3 stable branch

Created by: Razer6

5.3 stable branch is missing update docs from 5.2 to 5.3. This PR contains the fixed update docs from #4385

doc/update/5.2-to-5.3.md

+# From 5.2 to 5.3
+
+### 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
+
+### 2. Get latest code
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H git fetch
+sudo -u git -H git checkout 5-3-stable
+```
+
+### 3. 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
+```
+
+### 4. Update config files
+
+* Make `/home/git/gitlab/config/gitlab.yml` same as https://github.com/gitlabhq/gitlabhq/blob/5-3-stable/config/gitlab.yml.example but with your settings.
+* Make `/home/git/gitlab/config/puma.rb` same as https://github.com/gitlabhq/gitlabhq/blob/5-3-stable/config/puma.rb.example but with your settings.
+
+### 5. Update Init script
+
+```bash
+sudo rm /etc/init.d/gitlab
+sudo curl --output /etc/init.d/gitlab https://raw.github.com/gitlabhq/gitlabhq/5-3-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 (5.2)
+
+### 1. Revert the code to the previous version
+Follow the [`upgrade guide from 5.1 to 5.2`](5.1-to-5.2.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
+```
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