Create Gitlab-Integration-Service-for-Mattermost.md

doc/integrations/services/Gitlab-Integration-Service-for-Mattermost.md

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+# [GitLab Integration Service for Mattermost](https://github.com/mattermost/mattermost-integration-gitlab)
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+This [open source integration service](https://github.com/mattermost/mattermost-integration-gitlab) let you configure real-time notifications on GitLab issues, merge requests and comments to be delivered to selected Mattermost channels.
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+The service can be installed on any Linux-based web server and instructions for **Heroku** and **Ubuntu 14.04** are included. Please see [Mattermost incoming webhooks documentation](https://github.com/mattermost/platform/blob/master/doc/integrations/webhooks/Incoming-Webhooks.md) for details on formatting options within the service.
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+The Mattermost community is invited to fork, extend and repurpose this service for other applications. If you'd like your integration featured on http://mattermost.org/webhooks, please mail info@mattermost.org or tweet to us at @mattermosthq.
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+![webhooks](https://gitlab.com/gitlab-org/omnibus-gitlab/uploads/677b0aa055693c4dcabad0ee580c61b8/730_gitlab_feature_request.png)