Add version badge for Service Desk feature

doc/user/project/service_desk.md

@@ -110,6 +110,8 @@ in the email, `%{ISSUE_PATH}` placeholder which will be replaced by
 
 ### Using custom email display name
 
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/7529) in GitLab 12.8.
+
 You can customize the email display name. Emails sent from Service Desk will have
 this name in the `From` header. The default display name is `GitLab Support Bot`.