docs(readme): add Greenkeeper badge

README.md

@@ -7,7 +7,7 @@
 [![Codacy Badge](https://api.codacy.com/project/badge/Coverage/2554f9858cb742ed93eb22f49ccec3c3)](https://www.codacy.com/app/FullHuman/postcss-purgecss?utm_source=github.com&utm_medium=referral&utm_content=FullHuman/postcss-purgecss&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/postcss-purgecss.svg)](https://www.npmjs.com/package/@fullhuman/postcss-purgecss)
-[![license](https://img.shields.io/github/license/fullhuman/postcss-purgecss.svg)]()
+[![license](https://img.shields.io/github/license/fullhuman/postcss-purgecss.svg)]() [![Greenkeeper badge](https://badges.greenkeeper.io/FullHuman/postcss-purgecss.svg)](https://greenkeeper.io/)
 
 [PostCSS] plugin for PurgeCSS.