Update README.md to include Codemia in coding service section

Codemia is a website for practicing system design problems like how you practice algorithms and data structures on Leetcode/CodeForce.

Author: markqian

Diff for: README.md

@@ -258,4 +258,5 @@ But knowing the stuff will help you become better! :muscle:*
 - :link: [InterviewCake](https://www.interviewcake.com/)
 - :link: [At Coder](https://atcoder.jp/)
 - :link: [Codility](https://www.codility.com/)
-- :link: [ICPC Problem Archive](https://icpc.kattis.com/)
+- :link: [ICPC Problem Archive](https://icpc.kattis.com/)
+- :link: [Codemia](https://codemia.io/)