Merge branch 'connect_gitlab_buttons_doc' into 'master'

Connect GitLab buttons doc

The "GitLab buttons in Gmail" documentation was not linked from anywhere else.

Instead of assuming that these docs are not current, or should not be accessible, I've linked to them from doc/integrations/README.md.

If linking is the less desirable approach, please advise.

See merge request !339

doc/integration/README.md

@@ -8,6 +8,8 @@ See the documentation below for details on how to configure these services.
 - [LDAP](ldap.md) Set up sign in via LDAP
 - [OmniAuth](omniauth.md) Sign in via Twitter, GitHub, GitLab, and Google via OAuth.
 - [Slack](slack.md) Integrate with the Slack chat service
+- [OAuth2 provider](oauth_provider.md) OAuth2 application creation
+- [Gmail](gitlab_buttons_in_gmail.md) Adds GitLab actions to messages
 
 GitLab Enterprise Edition contains [advanced JIRA support](http://doc.gitlab.com/ee/integration/jira.html) and [advanced Jenkins support](http://doc.gitlab.com/ee/integration/jenkins.html).