Adding doc system to build. Updated docs, fixed links.

doc/integrations/Single-Sign-On/Gitlab.md

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+## Configuring GitLab Single-Sign-On
+
+The following steps can be used to configure Mattermost to use GitLab as a single-sign-on (SSO) service for team creation, account creation and sign-in.
+
+1. Login to your GitLab account and go to the Applications section either in Profile Settings or Admin Area.
+2. Add a new application called "Mattermost" with the following as Redirect URIs:
+  * `<your-mattermost-url>/login/gitlab/complete` (example: http://localhost:8065/login/gitlab/complete)
+  * `<your-mattermost-url>/signup/gitlab/complete`
+
+3. Submit the application and copy the given _Id_ and _Secret_ into the appropriate _SSOSettings_ fields in config/config.json
+
+4. Also in config/config.json, set _Allow_ to `true` for the _gitlab_ section, leave _Scope_ blank and use the following for the endpoints:
+  * _AuthEndpoint_: `<your-gitlab-url>/oauth/authorize` (example http://localhost:3000/oauth/authorize)
+  * _TokenEndpoint_: `<your-gitlab-url>/oauth/token` 
+  * _UserApiEndpoint_: `<your-gitlab-url>/api/v3/user`
+
+6. (Optional) If you would like to force all users to sign-up with GitLab only, in the _ServiceSettings_ section of config/config.json please set _DisableEmailSignUp_ to `true`.
+
+7. Restart your Mattermost server to see the changes take effect.