[Docs] add note about requiring babel-preset-airbnb and `airbnb-bro…

…wser-shims`.

Fixes #1544.

README.md

@@ -2,6 +2,8 @@
 
 *A mostly reasonable approach to JavaScript*
 
+> **Note**: this guide assumes you are using [Babel](https://babeljs.io), and requires that you use [babel-preset-airbnb](https://npmjs.com/babel-preset-airbnb) or the equivalent. It also assumes you are installing shims/polyfills in your app, with [airbnb-browser-shims](https://npmjs.com/airbnb-browser-shims) or the equivalent.
+
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