Part of class fields to stage 3 (#2908)

* Class fields to stage 3

Per 2017.07.27 TC39
https://github.com/tc39/proposals/commit/0bbbb08c3cdcc986d59b21782b70633329546042

* Be explicit that only the public part of the unified field proposal is available.

packages/react-scripts/template/README.md

@@ -199,7 +199,7 @@ In addition to [ES6](https://github.com/lukehoban/es6features) syntax features,
 * [Async/await](https://github.com/tc39/ecmascript-asyncawait) (ES2017).
 * [Object Rest/Spread Properties](https://github.com/sebmarkbage/ecmascript-rest-spread) (stage 3 proposal).
 * [Dynamic import()](https://github.com/tc39/proposal-dynamic-import) (stage 3 proposal)
-* [Class Fields and Static Properties](https://github.com/tc39/proposal-class-public-fields) (stage 2 proposal).
+* [Class Fields and Static Properties](https://github.com/tc39/proposal-class-public-fields) (part of stage 3 proposal).
 * [JSX](https://facebook.github.io/react/docs/introducing-jsx.html) and [Flow](https://flowtype.org/) syntax.
 
 Learn more about [different proposal stages](https://babeljs.io/docs/plugins/#presets-stage-x-experimental-presets-).