Add documentation for service desk email

doc/user/project/service_desk.md

@@ -95,18 +95,18 @@ directory in your repository. Commit and push to your default branch.
 
 The **Thank you email** is the email sent to a user after they submit an issue.
 The file name of the template has to be `thank_you.md`.
-You can use `%{ISSUE_ID}` placeholder which will be replaced by an issue iid in the email and
-`%{ISSUE_PATH}` placeholder which will be replaced by project path and the issue iid.
+You can use `%{ISSUE_ID}` placeholder which will be replaced by an issue IID in the email and
+`%{ISSUE_PATH}` placeholder which will be replaced by project path and the issue IID.
 As the service desk issues are created as confidential (only project members can see them)
-the response email doesn't provide the issue link.
+the response email does not provide the issue link.
 
 #### New note email
 
 The **New note email** is the email sent to a user when the issue they submitted has a new comment.
 The file name of the template has to be `new_note.md`.
-You can use `%{ISSUE_ID}` placeholder which will be replaced by an issue iid
+You can use `%{ISSUE_ID}` placeholder which will be replaced by an issue IID
 in the email, `%{ISSUE_PATH}` placeholder which will be replaced by
- project path and the issue iid and `%{NOTE_TEXT}` placeholder which will be replaced by the note text.
+ project path and the issue IID and `%{NOTE_TEXT}` placeholder which will be replaced by the note text.
 
 ### Using custom email display name
 
@@ -115,11 +115,66 @@ in the email, `%{ISSUE_PATH}` placeholder which will be replaced by
 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`.
 
+### Using custom email address
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/2201) in [GitLab Premium](https://about.gitlab.com/pricing/) 13.0.
+
+NOTE: **Note:**
+This feature is disabled by default. For steps to enable it, see [Enable custom email address](#enable-custom-email-address).
+
+If the `service_desk_email` feature flag is enabled in your configuration,
+then it's possible to create Service Desk issues by sending emails to the
+custom Service Desk email address, which should have the following format:
+`project_contact+%{key}@example.com`.
+
+The `%{key}` part is used to find the project where the issue should be created. The
+`%{key}` part combines the path to the project and configurable project name suffix:
+`<project_full_path>-<project_name_suffix>`.
+
+You can set the project name suffix in your project's Service Desk settings.
+It can contain only lowercase letters (`a-z`), numbers (`0-9`), or underscores (`_`).
+
+![Setting custom Service Desk email address](img/service_desk_custom_email_address_v13_0.png)
+
+For example, suppose you add the following to your configuration:
+
+```yaml
+service_desk_email:
+  enabled: true
+  address: "project_contact+%{key}@example.com"
+  user: "project_support@example.com"
+  password: "[REDACTED]"
+  host: "imap.gmail.com"
+  port: 993
+  ssl: true
+  start_tls: false
+  log_path: "log/mailroom.log"
+  mailbox: "inbox"
+  idle_timeout: 60
+```
+
+In this case, suppose the `mygroup/myproject` project Service Desk settings has the project name
+suffix set to `support`, and a user sends an email to `project_contact+mygroup-myproject-support@example.com`.
+As a result, a new Service Desk issue is created from this email in the `mygroup/myproject` project.
+
+#### Enable custom email address
+
+This feature comes with the `service_desk_email` feature flag disabled by default.
+To turn on the feature, ask a GitLab administrator with Rails console access to run the following
+command:
+
+```ruby
+Feature.enable(service_desk_email)
+```
+
+The configuration options are the same as for configuring
+[incoming email](../../administration/incoming_email.md#set-it-up).
+
 ## 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
+To create a Service Desk issue, an end user does not 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:
 
@@ -136,7 +191,7 @@ 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 usual. They'll see a familiar looking
+For responders to the issue, everything works as usual. They will see a familiar looking
 issue tracker, where they can see issues created via customer support requests and
 filter and interact with them just like other GitLab issues.