Add problemsets and competitive programming sites (#4075)

problem-sets-competitive-programming.md

@@ -23,6 +23,8 @@
 * [Coderbyte](https://coderbyte.com)
 * [Codewars](http://www.codewars.com)
 * [Codingame](https://www.codingame.com/start)
+* [COCI](https://hsin.hr/coci/)
+* [E-olymp](https://www.e-olymp.com/en/)
 * [Facebook Hackercup](https://www.facebook.com/hackercup)
 * [Google Code Jam](https://codingcompetitions.withgoogle.com/codejam)
 * [HackerEarth](https://www.hackerearth.com)
@@ -69,6 +71,7 @@
 * [Codeforces](http://codeforces.com/problemset)
 * [Codility](https://codility.com/programmers/)
 * [Coding Bat](http://codingbat.com/java)
+* [CSES Problem Set](https://cses.fi/problemset/)
 * [Exercism](http://exercism.io)
 * [Geeks For Geeks](http://www.geeksforgeeks.org)
 * [Google Code Jam - Practise](https://code.google.com/codejam/contests.html)