Update language support wording to ES2017

Author: Timer

packages/react-scripts/template/README.md

@@ -175,8 +175,8 @@ You don’t have to ever use `eject`. The curated feature set is suitable for sm
 
 ## Supported Language Features and Polyfills
 
-The project supports a superset of the latest JavaScript standard.  
-In addition to ES6, it also supports:
+This project supports a superset of the latest JavaScript standard, which includes async and await.  
+In addition to ES2017, it also supports:
 
 * [JSX](https://facebook.github.io/react/docs/introducing-jsx.html) and [Flow](https://flowtype.org/) syntax.
 * [Class Fields and Static Properties](https://github.com/tc39/proposal-class-public-fields) (stage 2 proposal).
@@ -192,7 +192,7 @@ Note that **the project only includes a few ES6 [polyfills](https://en.wikipedia
 * [`Promise`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) via [`promise`](https://github.com/then/promise).
 * [`fetch()`](https://developer.mozilla.org/en/docs/Web/API/Fetch_API) via [`whatwg-fetch`](https://github.com/github/fetch).
 
-If you use any other ES6 features that need **runtime support** (such as `Array.from()` or `Symbol`), make sure you are including the appropriate polyfills manually, or that the browsers you are targeting already support them.
+If you use any other ES6+ features that need **runtime support** (such as `Array.from()` or `Symbol`), make sure you are including the appropriate polyfills manually, or that the browsers you are targeting already support them.
 
 ## Syntax Highlighting in the Editor