Update README.md

Clarifying the fact that this is simply an integration for videojs-contrib-ads

README.md

@@ -24,7 +24,9 @@ Add "ads" and "vast" to the plugins object, and pass a url:
 
 And when you play that video, a pre-roll ad should play beforehand.
 
-Check out the [demo](https://github.com/theonion/videojs-vast-plugin/blob/master/example.html) for a more detailed example
+Check out the [demo](https://github.com/theonion/videojs-vast-plugin/blob/master/example.html) for a more detailed example.
+
+This plugin is technically an integration for video.js's [ads plugin](https://github.com/videojs/videojs-contrib-ads). For more information on the various states in the preroll process, please refer to the  [videojs-contrib-ads documentation](https://github.com/videojs/videojs-contrib-ads#state-diagram).
 
 ### Credit