added Kattis to competitive programming and problem sets (#2143)

problem-sets-competitive-programming.md

@@ -22,6 +22,7 @@
 * [Hello World Open](https://helloworldopen.com)
 * [Internet Problem Solving Contest](http://ipsc.ksp.sk)
 * [Just another Golf Coding](http://jagc.org)
+* [Kattis](https://open.kattis.com)
 * [Microcorruption](https://microcorruption.com/login)
 * [Saratov State University](http://acm.sgu.ru/index.php)
 * [Sphere Online Judge](http://www.spoj.com/contests)
@@ -51,6 +52,7 @@
 * [HackerEarth](https://www.hackerearth.com)
 * [HDU Online Judge](http://acm.hdu.edu.cn)
 * [InterviewBit](https://www.interviewbit.com)
+* [Kattis](https://open.kattis.com)
 * [Leetcode](https://leetcode.com)
 * [Mathproblem of the Month - Bilkent University](http://www.fen.bilkent.edu.tr/~cvmath/prob-month.html)
 * [PEG Judge](http://wcipeg.com)