Update Administration-guide.md

Author: Gargron

Running-Mastodon/Administration-guide.md

@@ -102,6 +102,8 @@ This will create a new user associated with the old account_id. They'll not have
 
 ## Activity monitoring
 
+Mastodon tracks some basic aggregated statistics about activity on your instance. This information is, by default, publicly available in the `/api/v1/instance/activitiy` API. It includes weekly unique logins, registrations and posted statuses. A "login" in this case means "using the website or API" rather than specifically using the login form.
+
 Munin graphs can be generated to track your instance activity.
 
 * https://github.com/cquest/mastodon-munin-plugins