Added comment to piwik.js about its usage

src/UI/piwik.js

@@ -9,3 +9,9 @@ if(window.NzbDrone.Analytics) {
     g.src = 'http://piwik.nzbdrone.com/piwik.js';
     s.parentNode.insertBefore(g, s);
 }
+
+// piwik is used to send anonymous information about your browser and which parts of the web interface you use to Sonarr servers.
+// We use this information to prioritize features and browser support.
+// We will NEVER include any personal information or any information that could identify you.
+//
+// You can opt out of this in general settings