Exercism edded in Practice Sites section

Author: Roman Kukharuk

README.md

@@ -159,3 +159,4 @@ But knowing the stuff will help you become better! :muscle:*
 - :link: [Spoj](https://spoj.com/)
 - :link: [HackerEarth](https://hackerearth.com/)
 - :link: [TopCoder](https://www.topcoder.com/)
+- :link: [Exercism](https://www.http://exercism.io/)