README: add ga beacon

Author: ebidel

README.md

@@ -15,3 +15,5 @@ Polymer is based on a set of future technologies, including [Shadow DOM](https:/
 ## Tools & Testing
 
 For running tests or building minified files, consult the [tooling information](http://polymer-project.org/tooling-strategy.html).
+
+[![Analytics](https://ga-beacon.appspot.com/UA-39334307-2/Polymer/polymer-dev/README)](https://github.com/igrigorik/ga-beacon)