Add new README file

README.md

+Gitlab Mattermost
+-----------------
+
+Gitlab Mattermost is a team communication service for sharing messages and
+files across PCs and phones, with archiving and instant search.
+
+This Gitlab project was created so Gitlab community members could file and 
+discuss issues regarding Gitlab Mattermost via: 
+https://gitlab.com/gitlab-org/gitlab-mattermost/issues
+
+Issues are monitored by the Mattermost team. 
+
+For security issues regarding Mattermost, please review our 
+[Responsible Disclosure Policy](http://www.mattermost.org/responsible-disclosure-policy/)
+