Add branches field description for mattermost

doc/user/project/integrations/mattermost.md

@@ -58,5 +58,6 @@ At the end, fill in your Mattermost details:
 | **Webhook**  | The incoming webhook URL which you have to set up on Mattermost, similar to: `http://mattermost.example/hooks/5xo…` |
 | **Username** | Optional username which can be on messages sent to Mattermost. Fill this in if you want to change the username of the bot. |
 | **Notify only broken pipelines** | If you choose to enable the **Pipeline** event and you want to be only notified about failed pipelines. |
+| **Branches to be notified** | Select which types of branches to send notifications for. |
 
 ![Mattermost configuration](img/mattermost_configuration.png)