Add framework for docs on Mattermost commands

[ci skip]

doc/project_services/mattermost_command.md

+# Mattermost Commands
+
+Mattermost commands give users an extra interface to perform common operations
+from the chat environment. This allows one to, for example, create an issue as
+soon as the idea was discussed in Mattermost.
+
+## Configuration
+
+### On Mattermost
+
+On Mattermost, an administrator has to enable custom slash commands. To do this,
+log on, and go to the system console.
+
+Click **Custom integrations**, and enable the custom slash commands and don't
+forget to save your settings.
+
+![Enable custom slash commands](img/mattermost_enable_custom_commands.png)
+
+Now go back to your team page, and go the the configuration page for a new
+slash command.
+
+**Integrations** > **Slash Command** > **Add Slash Command**
+
+### On GitLab
+
+On the project you want to configure the slash commands for, go to the
+Mattermost Command service. Under settings > **Project Services**.
+
+A screen will appear with all the values you can copy to the Mattermost page you
+have just opened.
+
+**Insert screenshot here**
+
+## Usage
+
+A users first interaction with the newly created slash commands will trigger an
+authorisation process. This process connects your Mattermost user with your
+GitLab user.
+
+After these steps are performed, you can start interacting with GitLab. Maybe
+start with `/your-trigger issue show 1` and see what your first issue was on
+this project!

doc/project_services/project_services.md

@@ -42,6 +42,7 @@ further configuration instructions and details. Contributions are welcome.
 | [Irker (IRC gateway)](irker.md) | Send IRC messages, on update, to a list of recipients through an Irker gateway |
 | [JIRA](jira.md) | JIRA issue tracker |
 | JetBrains TeamCity CI | A continuous integration and build server |
+| [Mattermost Commands](mattermost_command.md) | Mattermost slash commands |
 | PivotalTracker | Project Management Software (Source Commits Endpoint) |
 | Pushover | Pushover makes it easy to get real-time notifications on your Android device, iPhone, iPad, and Desktop |
 | [Redmine](redmine.md) | Redmine issue tracker |