Close #539 PR: Add CSS Protips.

readme.md

@@ -125,6 +125,7 @@ Check out my [blog](https://blog.sindresorhus.com) and follow me on [Twitter](ht
 	- [Critical-Path (Above-the-fold) Tools](https://github.com/addyosmani/critical-path-css-tools)
 	- [Scalability](https://github.com/davidtheclark/scalable-css-reading-list)
 	- [Must-Watch Talks](https://github.com/AllThingsSmitty/must-watch-css)
+	- [Protips](https://github.com/AllThingsSmitty/css-protips)
 - [React](https://github.com/enaqx/awesome-react)
 	- [Relay](https://github.com/expede/awesome-relay)
 - [Web Components](https://github.com/mateusortiz/webcomponents-the-right-way)