Add description

Author: zackbloom

README.md

@@ -5,6 +5,10 @@ Include [pace.js](https://raw.github.com/HubSpot/pace/v0.4.2/pace.min.js) and th
 [theme](http://github.hubspot.com/pace/docs/themes/) css of your choice on your page
 (as early as is possible), and you're done!
 
+Pace will automatically monitor your ajax requests, event loop lag, document
+ready state, and elements on your page to decide the progress.  On ajax navigation
+it will begin again!
+
 If you use AMD or Browserify, require in pace.js and call `pace.start()` as early in
 the loading process as is possible.