Add HackerEarth to list of challenge sites

README.md

@@ -1400,6 +1400,7 @@ Challenge sites:
 - [TopCoder](https://www.topcoder.com/)
 - [Project Euler (math-focused)](https://projecteuler.net/index.php?section=problems)
 - [Codewars](http://www.codewars.com)
+- [HackerEarth](https://www.hackerearth.com/)
 - [HackerRank](https://www.hackerrank.com/)
 - [Codility](https://codility.com/programmers/)
 - [InterviewCake](https://www.interviewcake.com/)