Add documentation about buttons in gmail.

doc/integration/gitlab_buttons_in_gmail.md

+# GitLab buttons in gmail
+
+GitLab supports [Google actions in email](https://developers.google.com/gmail/markup/actions/actions-overview).
+
+If correctly setup, emails that require an action will be marked in Gmail.
+
+![gitlab_actions](gitlab_actions.png)
+
+To get this functioning, you need to be registered with Google.
+[See how to register with google in this document.](https://developers.google.com/gmail/markup/registering-with-google)
+