docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # cz-conventional-changelog
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/commitizen/cz-conventional-changelog.svg)](https://greenkeeper.io/)
+
 Status:
 [![npm version](https://img.shields.io/npm/v/cz-conventional-changelog.svg?style=flat-square)](https://www.npmjs.org/package/cz-conventional-changelog)
 [![npm downloads](https://img.shields.io/npm/dm/cz-conventional-changelog.svg?style=flat-square)](http://npm-stat.com/charts.html?package=cz-conventional-changelog&from=2015-08-01)