Keep update docs inside repo so people can submit PR

doc/update/5.0-to-5.1.md

+## Release notes:
+
+* `unicorn` replaced with `puma`
+* merge request cached diff will be truncated
+
+### 1. stop server
+
+    sudo service gitlab stop
+
+### 2. get latest code
+
+```
+
+sudo -u git -H git fetch
+sudo -u git -H git checkout 5-1-stable
+
+```
+
+### 3. Install libs, migrations etc
+
+```
+sudo -u git -H cp config/puma.rb.example config/puma.rb
+
+
+sudo -u git -H bundle install --without development test postgres --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
+
+```
+
+### 4. Update init.d script with a new one
+
+```bash
+# init.d
+sudo rm /etc/init.d/gitlab
+sudo curl --output /etc/init.d/gitlab https://raw.github.com/gitlabhq/gitlab-recipes/5-1-stable/init.d/gitlab
+sudo chmod +x /etc/init.d/gitlab
+```
+
+### 5. Start application
+
+    sudo service gitlab start