Extended JavaScript environment requirements polyfills list with a promise (reactjs#1553)

Author: Kazimirkas

content/docs/reference-javascript-environment-requirements.md

@@ -11,6 +11,7 @@ React 16 depends on the collection types [Map](https://developer.mozilla.org/en-
 A polyfilled environment for React 16 using core-js to support older browsers might look like:
 
 ```js
+import 'core-js/es6/promise';
 import 'core-js/es6/map';
 import 'core-js/es6/set';