Merge branch 'zj-mattermost-auto-config-docs' into 'master'

Docs on MM auto config

See merge request !8230

doc/project_services/mattermost_slash_commands.md

@@ -14,12 +14,18 @@ If you have the Omnibus GitLab package installed, Mattermost is already bundled
 in it. All you have to do is configure it. Read more in the
 [Omnibus GitLab Mattermost documentation][omnimmdocs].
 
-## Configuration
+## Automated Configuration
+
+If Mattermost is installed on the same server as GitLab, the configuration process can be
+done for you by GitLab. 
+
+Go to the Mattermost Slash Command service on your project and click the 'Add to Mattermost' button.
+
+## Manual Configuration
 
 The configuration consists of two parts. First you need to enable the slash
 commands in Mattermost and then enable the service in GitLab.
 
-
 ### Step 1. Enable custom slash commands in Mattermost
 
 This step is only required when using a source install, omnibus installs will be