Add twitter documentation.

doc/README.md

@@ -11,7 +11,7 @@
 **Administrator documentation**
 
 + [Install](install/README.md) Requirements, directory structures and manual installation.
-+ [Integration](integration/external-issue-tracker.md) How to integrate JIRA and Redmine.
++ [Integration](integration/README.md) How to integrate with systems such as JIRA, Redmine, LDAP and Twitter.
 + [Raketasks](raketasks/README.md) Explore what GitLab has in store for you to make administration easier.
 + [System hooks](system_hooks/system_hooks.md) Let GitLab notify you when certain management tasks need to be carried out.
 + [Security](security/README.md) Learn what you can do to further secure your GitLab instance.

doc/integration/README.md

++ [External issue tracker](external-issue-tracker.md)
++ [LDAP](ldap.md)
++ [Twitter](twitter.md)

doc/integration/twitter_oauth.md

+You can use Twitter to log into GitLab.
+
+Please configure this in gitlab.yml
+
+Below are screenshots how to setup your app on Twitter for this.
+
+![Application details](twitter_app_details.png)
+![API Keys](twitter_app_api_keys.png)
+