Added info about matomo IP db.

ops/docker-compose/matomo/README.md

@@ -3,4 +3,7 @@ database. This is currently not used by the official nomad production deployment
 
 The example proxy rules use a path that does not hint at matomo/piwik and that rewrite
 key files (piwik.js, piwik.php) that cause many tracking events beeing blocked by
-ad-blocker or similar browser plug-ins.
\ No newline at end of file
+ad-blocker or similar browser plug-ins.
+
+We use a mount to add a IP database to matomo. The IP db is downloaded from:
+https://download.db-ip.com/free/dbip-city-lite-2020-07.mmdb.gz