docs(readme): add Greenkeeper badge

https://greenkeeper.io/

Author: greenkeeper[bot]

readme.md

@@ -1,5 +1,7 @@
 # [Vanilla CSS framework](http://gitscrum.github.io/slims)
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/GitScrum/slims.svg)](https://greenkeeper.io/)
+
 > The ideology of methodology atomic
 
 [![Travis Build Status](https://img.shields.io/travis/gitscrum/slims.svg?style=flat-square&label=unix)](https://travis-ci.org/gitscrum/slims)[![AppVeyor Build Status](https://img.shields.io/appveyor/ci/gitscrum/slims.svg?style=flat-square&label=windows)](https://ci.appveyor.com/project/gitscrum/slims)![](https://img.shields.io/badge/stylelint-passing-green.svg?style=flat-square)![node](https://img.shields.io/node/v/slims.svg?maxAge=2592000&style=flat-square)[![npm version](https://img.shields.io/npm/v/slims.svg?style=flat-square)](https://www.npmjs.com/package/slims)[![devDependency Status](https://david-dm.org/gitscrum/slims/dev-status.svg?style=flat-square)](https://david-dm.org/gitscrum/Slims#info=devDependencies)[![Coveralls status](https://img.shields.io/coveralls/gitscrum/slims.svg?style=flat-square)](https://coveralls.io/r/gitscrum/slims)