Update guide for 6.3 to 6.4

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

doc/update/6.3-to-6.4.md

+# From 6.3 to 6.4
+
+### 0. Backup
+
+```bash
+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. Get latest code
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H git fetch
+sudo -u git -H git checkout 6-4-stable
+```
+
+### 3. Update gitlab-shell (and its config)
+
+```bash
+cd /home/git/gitlab-shell
+sudo -u git -H git fetch
+sudo -u git -H git checkout v1.8.0
+```
+
+### 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
+
+
+# Run database migrations
+sudo -u git -H bundle exec rake db:migrate RAILS_ENV=production
+
+# Clean up assets and cache
+sudo -u git -H bundle exec rake assets:clean assets:precompile cache:clear RAILS_ENV=production
+```
+
+### 5. Start application
+
+    sudo service gitlab start
+    sudo service nginx restart
+
+### 8. 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 (6.3)
+
+### 1. Revert the code to the previous version
+Follow the [`upgrade guide from 6.2 to 6.3`](6.2-to-6.3.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 bundle exec rake gitlab:backup:restore RAILS_ENV=production
+```