docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -12,6 +12,8 @@ A [clean-css](https://github.com/jakubpawlowicz/clean-css) loader for [webpack](
 
 # Clean-CSS-loader [![Build Status](https://travis-ci.org/retyui/clean-css-loader.svg?branch=master)](https://travis-ci.org/retyui/clean-css-loader) [![npm version](https://badge.fury.io/js/clean-css-loader.svg)](https://badge.fury.io/js/clean-css-loader) [![dependencies Status](https://david-dm.org/retyui/clean-css-loader/status.svg)](https://david-dm.org/retyui/clean-css-loader)
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/retyui/clean-css-loader.svg)](https://greenkeeper.io/)
+
 
 ## Install
 `npm i -D clean-css-loader` <em>or</em> `yarn add clean-css-loader --dev`