Added Codewars as a code challenge resource

Not a complete list without Codewars!

README.md

@@ -1304,6 +1304,7 @@ Take coding challenges every day, as many as you can.
     - [LeetCode](https://leetcode.com/)
     - [TopCoder](https://www.topcoder.com/)
     - [Project Euler (math-focused)](https://projecteuler.net/index.php?section=problems)
+    - [Codewars](http://www.codewars.com)
     - [HackerRank](https://www.hackerrank.com/)
     - [Codility](https://codility.com/programmers/)
     - [InterviewCake](https://www.interviewcake.com/)