forum: document topic creating by email

service/forum.md

@@ -12,6 +12,23 @@ benefit of the Tor community.
 
 # How-to
 
+## Enable new topics by email
+
+Topic creation by email is the ability to create a new forum topic in a category
+simply by sending an email to a specific address.
+
+This feature is enabled per-category. To enable it for a category, navigate to
+it, click the wrench icon (top right), open the **Settings** tab and scroll to
+the **Email** header. Check the box next to `Accept incoming emails sent to:`.
+
+The email address is managed automatically by Discourse, there's no need to
+create it. However, the address doesn't automatically appear on the forum so to
+allow users to discover it, it's a good idea to add it to the category
+description.
+
+Per the forum's settings, only users with trust level 2 (member) or higher are
+allowed to post new topics by email.
+
 ## Use the app
 
 The official companion app for Discourse is [DiscourseHub][]