Added an additional very popular resource

Author: dasAtRagedy

coding-competitions.md

@@ -13,3 +13,4 @@ Competitons to participate in when you are bored, or just want to code!
 - [Leetcode](https://leetcode.com/)
 - [Codingame](https://www.codingame.com/)
 - [SPOJ](https://www.spoj.com/)
+- [Codeforces](https://codeforces.com/)