Add docs on how to configure email notifications for abuse reports

I've had to find this the hard way once myself, and it came up in
questions in Slack recently as well.  It is mentioned in passing at
user/admin_area/settings/#reporting but IMO could be explained much more
explicitly

doc/user/admin_area/abuse_reports.md

@@ -12,6 +12,14 @@ View and resolve abuse reports from GitLab users.
 GitLab administrators can view and [resolve](#resolving-abuse-reports) abuse
 reports in the Admin Area.
 
+## Receiving notifications of abuse reports
+
+To receive notifications of new abuse reports by e-mail, follow these steps:
+
+1. Select **{admin}** **Admin Area > Settings >  Reporting**.
+1. Expand the **Abuse reports** section.
+1. Provide an email address.
+
 ## Reporting abuse
 
 To find out more about reporting abuse, see [abuse reports user