adding koding.com to list of tools

Author: sent-hil

README.md

@@ -75,6 +75,7 @@ Some of these links are affiliate links, meaning that if you make a purchase, I
 * [Slack](http://slack.com) Chat for teams, with GitHub and Google hangouts integration
 * [PrettyDiff](http://prettydiff.com/)
 * [Babel Repl](https://babeljs.io/repl) The Babel REPL with compiled output
+* [Koding](https://koding.com) Online (free) cloud development platform with video & audio collaboration
 
 
 ## Building