Mention about ES2015 in README

Author: voter101

README.md

@@ -1,6 +1,6 @@
 # React.js Koans
 
-If you want to learn React.js you came to the right place. We prepared a set of practical exercises that will help you learn React.js from square one. The only thing you need to know is JavaScript.
+If you want to learn React.js you came to the right place. We prepared a set of practical exercises that will help you learn React.js from square one. The only thing you need to know is JavaScript. Here we are using [ECMAScript 2015 standard](https://babeljs.io/docs/learn-es2015/).
 
 The Koans are a set of tasks to complete. Prepared tests checks if they are done correctly.