docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -8,7 +8,7 @@
 [![Codacy Badge](https://api.codacy.com/project/badge/Coverage/c23bd13d30104a7a89bed239166aaf69)](https://www.codacy.com/app/FullHuman/purgecss-webpack-plugin?utm_source=github.com&utm_medium=referral&utm_content=FullHuman/purgecss-webpack-plugin&utm_campaign=Badge_Coverage)
 [![styled with prettier](https://img.shields.io/badge/styled_with-prettier-ff69b4.svg)](https://github.com/prettier/prettier)
 [![npm](https://img.shields.io/npm/v/purgecss-webpack-plugin.svg)](https://www.npmjs.com/package/purgecss-webpack-plugin)
-[![license](https://img.shields.io/github/license/fullhuman/purgecss-webpack-plugin.svg)]()
+[![license](https://img.shields.io/github/license/fullhuman/purgecss-webpack-plugin.svg)]() [![Greenkeeper badge](https://badges.greenkeeper.io/FullHuman/purgecss-webpack-plugin.svg)](https://greenkeeper.io/)
 
 [Webpack](https://github.com/webpack/webpack) plugin to remove unused css.