Add Google Kickstart to problem-sets-competitive-programming.md (#4742)

* Add Google Kickstart to problem-sets-competitive-programming.md

* alphabetize

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

problem-sets-competitive-programming.md

@@ -29,6 +29,7 @@
 * [E-olymp](https://www.e-olymp.com/en/)
 * [Facebook Hackercup](https://www.facebook.com/hackercup)
 * [Google Code Jam](https://codingcompetitions.withgoogle.com/codejam)
+* [Google Kickstart](https://codingcompetitions.withgoogle.com/kickstart)
 * [HackerEarth](https://www.hackerearth.com)
 * [Hackerrank](https://www.hackerrank.com)
 * [Internet Problem Solving Contest](http://ipsc.ksp.sk)