docs: add CSSBattle

Author: AlexsandroSA

README.md

@@ -15,6 +15,7 @@
  - [Frontend Mentor](https://www.frontendmentor.io/challenges)
  - [30 Days of Postman](https://blog.postman.com/introducing-30-days-of-postman-coding-challenge/)
  - [Codepen challenges](https://codepen.io/challenges)
+ - [CSSBattle](https://cssbattle.dev/)
 
 ## Solving Problems :dart: