Add Slack slash commands to project services list index

[ci skip]

doc/project_services/project_services.md

@@ -46,6 +46,7 @@ further configuration instructions and details. Contributions are welcome.
 | [Mattermost slash commands](mattermost_slash_commands.md) | Mattermost chat and ChatOps slash commands |
 | [Mattermost Notifications](mattermost.md) | Receive event notifications in Mattermost |
 | [Slack Notifications](slack.md) | Receive event notifications in Slack |
+| [Slack slash commands](slack_slash_commands.md) | Slack chat and ChatOps 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 |