docs(readme): add Greenkeeper badge

https://greenkeeper.io/

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # documentation-theme-utils
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/documentationjs/documentation-theme-utils.svg)](https://greenkeeper.io/)
+
 [![Circle CI](https://circleci.com/gh/documentationjs/documentation-theme-utils/tree/master.svg?style=svg)](https://circleci.com/gh/documentationjs/documentation-theme-utils/tree/master)
 [![Coverage Status](https://coveralls.io/repos/documentationjs/documentation-theme-utils/badge.svg?branch=master&service=github)](https://coveralls.io/github/documentationjs/documentation-theme-utils?branch=master)