Add screenshot in docs

[ci skip]

doc/project_services/mattermost_command.md → doc/project_services/mattermost_slash_commands.md

@@ -29,7 +29,7 @@ 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**
+![Mattermost setup instructions](img/mattermost_config_help.png)
 
 ## Usage
 

doc/project_services/project_services.md

@@ -42,7 +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 |
+| [Mattermost Slash Commands](mattermost_slash_commands.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 |