docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -5,6 +5,7 @@
 [![jsDelivr CDN](https://data.jsdelivr.com/v1/package/npm/jQuery-QueryBuilder/badge)](https://www.jsdelivr.com/package/npm/jQuery-QueryBuilder)
 [![Build Status](https://img.shields.io/travis/mistic100/jQuery-QueryBuilder.svg?style=flat-square)](https://travis-ci.org/mistic100/jQuery-QueryBuilder)
 [![Coverage Status](https://img.shields.io/coveralls/mistic100/jQuery-QueryBuilder/master.svg?style=flat-square)](https://coveralls.io/r/mistic100/jQuery-QueryBuilder)
+[![Dependencies Status](https://david-dm.org/mistic100/jQuery-QueryBuilder/status.svg?style=flat-square)](https://david-dm.org/mistic100/jQuery-QueryBuilder)
 
 jQuery plugin offering an simple interface to create complex queries.