add XO badge

readme.md

@@ -9,7 +9,7 @@
 
 > Terminal string styling done right
 
-[![Build Status](https://travis-ci.org/chalk/chalk.svg?branch=master)](https://travis-ci.org/chalk/chalk) [![Coverage Status](https://coveralls.io/repos/github/chalk/chalk/badge.svg?branch=master)](https://coveralls.io/github/chalk/chalk?branch=master) [![](https://img.shields.io/badge/unicorn-approved-ff69b4.svg)](https://www.youtube.com/watch?v=9auOCbH5Ns4)
+[![Build Status](https://travis-ci.org/chalk/chalk.svg?branch=master)](https://travis-ci.org/chalk/chalk) [![Coverage Status](https://coveralls.io/repos/github/chalk/chalk/badge.svg?branch=master)](https://coveralls.io/github/chalk/chalk?branch=master) [![](https://img.shields.io/badge/unicorn-approved-ff69b4.svg)](https://www.youtube.com/watch?v=9auOCbH5Ns4) [![XO code style](https://img.shields.io/badge/code_style-XO-5ed9c7.svg)](https://github.com/sindresorhus/xo)
 
 [colors.js](https://github.com/Marak/colors.js) used to be the most popular string styling module, but it has serious deficiencies like extending `String.prototype` which causes all kinds of [problems](https://github.com/yeoman/yo/issues/68). Although there are other ones, they either do too much or not enough.