Add competitive programming website (#4211)

* Add competitive programming website

* alphabetize

Co-authored-by: Eric Hellman <eric@hellman.net>

problem-sets-competitive-programming.md

@@ -21,9 +21,11 @@
 * [Codeeval](https://www.codeeval.com)
 * [CodeFights](https://codefights.com)
 * [Codeforces](http://codeforces.com/contests)
+* [Codeground](https://www.codeground.org)
 * [Coderbyte](https://coderbyte.com)
 * [Codewars](http://www.codewars.com)
 * [Codingame](https://www.codingame.com/start)
+* [DMOJ](https://dmoj.ca)
 * [E-olymp](https://www.e-olymp.com/en/)
 * [Facebook Hackercup](https://www.facebook.com/hackercup)
 * [Google Code Jam](https://codingcompetitions.withgoogle.com/codejam)
@@ -34,6 +36,7 @@
 * [Kattis](https://open.kattis.com)
 * [LeetCode](https://leetcode.com)
 * [Microcorruption](https://microcorruption.com/login)
+* [oj.uz](https://oj.uz)
 * [Sphere Online Judge](http://www.spoj.com/contests)
 * [Topcoder](https://www.topcoder.com)
 * [Toph](https://toph.co)