Handle several RDBMS

doc/update/5.0-to-5.1.md

@@ -21,8 +21,12 @@ sudo -u git -H git checkout 5-1-stable
 ```
 sudo -u git -H cp config/puma.rb.example config/puma.rb
-
+# Mysql users:
 sudo -u git -H bundle install --without development test postgres --deployment
+
+# Postgres users:
+sudo -u git -H bundle install --without development test mysql --deployment
+
 sudo -u git -H bundle exec rake db:migrate RAILS_ENV=production
 sudo -u git -H bundle exec rake migrate_merge_requests RAILS_ENV=production