Add LeetCode into coding practiсe section

Author: ismagilov

README.md

@@ -166,3 +166,5 @@ But knowing the stuff will help you become better! :muscle:*
 - :link: [Exercism](http://www.exercism.io/)
 - :link: [CodeFights](https://codefights.com/)
 - :link: [Project Euler](https://projecteuler.net/)
+- :link: [LeetCode](https://leetcode.com/)
+