Add Master CSS (#2382)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

readme.md

@@ -421,6 +421,7 @@
 - [Design systems](https://github.com/klaufel/awesome-design-systems#readme) - Collection of reusable components, guided by rules that ensure consistency and speed.
 - [Inertia.js](https://github.com/innocenzi/awesome-inertiajs#readme) - Make single-page apps without building an API.
 - [MDBootstrap](https://github.com/mdbootstrap/awesome-mdbootstrap#readme) - Templates, layouts, components, and widgets to rapidly build websites.
+- [Master CSS](https://github.com/master-co/awesome-master-css#readme) - A virtual CSS language with enhanced syntax.
 - [Hydrogen](https://github.com/shopify/awesome-hydrogen#readme) - Edge-first framework for building Shopify storefronts with React.
 
 ## Back-End Development