docs(readme): add Greenkeeper badge

greenkeeper[bot] · greenkeeper[bot] · commit a078d01f044b · 2018-07-13T03:58:26.000Z
diff --git a/README.md b/README.md
@@ -1,5 +1,7 @@
 # CSS Coverage!
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/philschatz/css-coverage.js.svg)](https://greenkeeper.io/)
+
 Generates coverage information of your CSS files and creates reports using the optional source maps.
 
 ## How is this different from other CSS coverage tools?
