docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # bittrex-typescript client
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/lookfirst/bittrex-typescript.svg)](https://greenkeeper.io/)
+
 [![npm version](https://badge.fury.io/js/bittrex-typescript.svg)](https://badge.fury.io/js/bittrex-typescript)
 [![Dependency Status](https://david-dm.org/boennemann/badges.svg)](https://david-dm.org/boennemann/badges)
 [![Build Status](https://david-dm.org/boennemann/badges.svg)](https://travis-ci.org/lookfirst/bittrex-typescript)