docs(readme): add Greenkeeper badge

README.md

@@ -4,7 +4,7 @@
 
 [![Build Status](https://travis-ci.com/lukechilds/vainjs.svg?branch=master)](https://travis-ci.com/lukechilds/vainjs)
 [![Coverage Status](https://coveralls.io/repos/github/lukechilds/vainjs/badge.svg?branch=master)](https://coveralls.io/github/lukechilds/vainjs?branch=master)
-[![npm](https://img.shields.io/npm/v/vainjs.svg)](https://www.npmjs.com/package/vainjs)
+[![npm](https://img.shields.io/npm/v/vainjs.svg)](https://www.npmjs.com/package/vainjs) [![Greenkeeper badge](https://badges.greenkeeper.io/lukechilds/vainjs.svg)](https://greenkeeper.io/)
 
 Generates vanity addresses for a wide range of address formats.