Add Free tier badge

Add Free tier badge to Incoming email page.

doc/administration/incoming_email.md

@@ -4,7 +4,7 @@ group: Project Management
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
 ---
 
-# Incoming email
+# Incoming email **(FREE SELF)**
 
 GitLab has several features based on receiving incoming emails: