added link to HackerEarth (#2174)

problem-sets-competitive-programming.md

@@ -19,6 +19,7 @@
 * [Codingame](https://www.codingame.com/start)
 * [Facebook Hackercup](https://www.facebook.com/hackercup)
 * [Hackerrank](https://www.hackerrank.com)
+* [HackerEarth](https://www.hackerearth.com)
 * [Hello World Open](https://helloworldopen.com)
 * [Internet Problem Solving Contest](http://ipsc.ksp.sk)
 * [Just another Golf Coding](http://jagc.org)