Added Codeforces link in README.md

Author: kushagra1212

README.md

@@ -19,6 +19,7 @@
 - https://www.codechef.com/
 - https://www.spoj.com/
 - https://www.urionlinejudge.com.br/
+- https://codeforces.com/
 
 # Javascript
 - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide