docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,4 +1,6 @@
 [![Build Status](https://semaphoreci.com/api/v1/nodeos/nodejs/branches/master/badge.svg)](https://semaphoreci.com/nodeos/nodejs)
 
 # nodejs
+
+[![Greenkeeper badge](https://badges.greenkeeper.io/NodeOS/nodejs.svg)](https://greenkeeper.io/)
 Node.js as a npm package for NodeOS