Adding link to GitLab Mattermost MIT License

README.md

@@ -47,4 +47,6 @@ https://gitlab.com/gitlab-org/gitlab-mattermost/issues).
 *Screenshot: Preview of Slack Import feature available in GitLab 8.0 release* 
-![alt text](http://www.mattermost.org/wp-content/uploads/2015/09/slack_import_600.png)
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+![alt text](http://www.mattermost.org/wp-content/uploads/2015/09/slack_import_600.png)
+
+GitLab Mattermost is available under an [MIT license](https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/config/software/mattermost.rb) as a compiled version of the [Mattermost open source project](http://www.mattermost.org/).
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