Add docs for Service Desk

doc/user/project/service_desk.md

+# Service Desk
+
+> [Introduced][ee-149] in [GitLab Enterprise Edition Premium][ee] 9.1.
+
+GitLab Service Desk is a simple way to allow people to create issues in your
+GitLab instance without needing their own user account.
+
+It provides a unique email address for end users to create issues in a project,
+and replies can be sent either through the GitLab interface or by email. End
+users will only see the thread through email.
+
+## Configuring Service Desk
+
+1.   [Set up reply by email][reply-by-email] for the GitLab instance. This must
+     support [email sub-addressing][email-sub-addressing].
+2.   As an administrator user on your GitLab instance, go to a project's settings
+     page.
+3.   If you have the correct access and an Enterprise Edition Premium license,
+     you will see an option to set up Service Desk:
+
+     ![Activate Service Desk option](img/service_desk_disabled.png)
+4.   Checking that box will enable Service Desk for the project, and show a
+     unique email address to email issues to the project. These issues will be
+     [confidential], so they will only be visible to project members.
+
+     **Warning**: as the screenshot below shows, this email address can be used
+     by anyone to create an issue on this project, whether or not they have
+     access to your GitLab instance. We recommend **putting this behind an
+     alias** so that it can be changed if needed, and
+     **[enabling Akismet][akismet]** on your GitLab instance to add spam
+     checking to this service.
+
+     ![Service Desk enabled](img/service_desk_enabled.png)
+5.   Service Desk is now enabled for this project!
+
+## Using Service Desk
+
+### As an end user (issue creator)
+
+To create a Service Desk issue, an end user doesn't need to know anything about
+the GitLab instance. They just send an email to the address they are given, and
+receive an email back confirming receipt:
+
+![Service Desk enabled](img/service_desk_confirmation_email.png)
+
+This also gives the end user an option to unsubscribe.
+
+If they don't choose to unsubscribe, then any new comments added to the issue
+will be sent as emails:
+
+![Service Desk reply email](img/service_desk_reply.png)
+
+And any responses they send will be displayed in the issue itself.
+
+### As a responder to the issue
+
+For responders to the issue, everything works as normal. Messages from the end
+user will show as coming from the special Support Bot user, but apart from that,
+you can read and write comments as normal:
+
+![Service Desk issue thread](img/service_desk_thread.png)
+
+[ee-149]: https://gitlab.com/gitlab-org/gitlab-ee/issues/149 "Service Desk with email"
+[ee]: https://about.gitlab.com/gitlab-ee/ "GitLab Enterprise Edition landing page"
+[reply-by-email]: ../../administration/reply_by_email.md#set-it-up
+[email-sub-addressing]: ../../administration/reply_by_email.md#email-sub-addressing
+[confidential]: ./issues/confidential_issues.md "Confidential issues"
+[akismet]: ../../integration/akismet.md

doc/workflow/README.md

@@ -42,6 +42,7 @@
   - (EE) [Merge request approvals](../user/project/merge_requests/merge_request_approvals.md)
   - (EE) [Fast-forward merge requests](../user/project/merge_requests/fast_forward_merge.md)
 - (EE) [Repository mirroring](repository_mirroring.md)
+- (EE Premium) [Service Desk](../user/project/service_desk.md)
 - [Manage large binaries with Git LFS](lfs/manage_large_binaries_with_git_lfs.md)
 - [Importing from SVN, GitHub, Bitbucket, etc](importing/README.md)
 - [Todos](todos.md)