Document Flow support (facebook#1384)

* Document Flow support

Projects created by Create React App use the `babel-preset-react` which includes
Flow babel plugins which introduces new syntax. This is important for users to know
as it causes what used to be syntax errors to be silently suppressed because they're
valid type annotations in Flow. For example if a user accidentally has `[foo: 'bar']`,
Babel will silently convert it to `[foo]`.

* Make it a bit clearer

README.md

@@ -145,7 +145,7 @@ Please refer to the [User Guide](https://github.com/facebookincubator/create-rea
 
 **If you’re getting started** with React, use `create-react-app` to automate the build of your app. There is no configuration file, and `react-scripts` is the only extra build dependency in your `package.json`. Your environment will have everything you need to build a modern React app:
 
-* React, JSX, and ES6 support.
+* React, JSX, ES6, and Flow syntax support.
 * Language extras beyond ES6 like the object spread operator.
 * A dev server that lints for common errors.
 * Import CSS and image files directly from JavaScript.