docs(readme): add Greenkeeper badge

https://greenkeeper.io/

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # jQuery QueryBuilder
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/mistic100/jQuery-QueryBuilder.svg)](https://greenkeeper.io/)
+
 [![Bower version](https://img.shields.io/bower/v/jQuery-QueryBuilder.svg?style=flat-square)](http://querybuilder.js.org)
 [![npm version](https://img.shields.io/npm/v/jQuery-QueryBuilder.svg?style=flat-square)](https://www.npmjs.com/package/jQuery-QueryBuilder)
 [![CDN](https://img.shields.io/badge/cdn-jsdelivr-%23EB4C36.svg?style=flat-square)](http://www.jsdelivr.com/projects/jquery.query-builder)