Update README.md

add a coveralls.io link

README.md

@@ -3,6 +3,7 @@
 ![Stagehand banner](https://raw.githubusercontent.com/google/stagehand/master/site/banner_stagehand.jpg)
 
 [![Build Status](https://travis-ci.org/google/stagehand.svg?branch=master)](https://travis-ci.org/google/stagehand)
+[![Coverage Status](https://coveralls.io/repos/google/stagehand/badge.svg?branch=master)](https://coveralls.io/r/google/stagehand?branch=master)
 
 ## Helps you get setup!