Mention related package

Author: sindresorhus

readme.md

@@ -32,5 +32,6 @@ Or with [`sass-loader`](https://github.com/webpack-contrib/sass-loader):
 
 ## Related
 
+- [css-extras](https://github.com/sindresorhus/css-extras) - Useful CSS custom functions
 - [react-extras](https://github.com/sindresorhus/react-extras) - Useful components and utilities for working with React
 - [modern-normalize](https://github.com/sindresorhus/modern-normalize) - Normalize browsers' default style