docs(readme): add Greenkeeper badge

https://greenkeeper.io/

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # zeromq
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/zeromq/zeromq.js.svg)](https://greenkeeper.io/)
+
 [![codecov](https://codecov.io/gh/zeromq/zeromq.js/branch/master/graph/badge.svg)](https://codecov.io/gh/zeromq/zeromq.js)
 [![](https://img.shields.io/badge/version-latest-blue.svg)](https://github.com/zeromq/zeromq.js)
 [![Build Status](https://travis-ci.org/zeromq/zeromq.js.svg?branch=master)](https://travis-ci.org/zeromq/zeromq.js)