Add docs about browsers list post processing CSS

Author: Timer

packages/react-scripts/template/README.md

@@ -596,6 +596,10 @@ This will allow you to do imports like
 
 This project setup minifies your CSS and adds vendor prefixes to it automatically through [Autoprefixer](https://github.com/postcss/autoprefixer) so you don’t need to worry about it.
 
+Support for new CSS features like the [`all` property](https://developer.mozilla.org/en-US/docs/Web/CSS/all), [`break` properties](https://www.w3.org/TR/css-break-3/#breaking-controls), [custom properties](https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_variables), and [media query ranges](https://www.w3.org/TR/mediaqueries-4/#range-context) are automatically polyfilled to add support for older browsers.
+
+You can customize your target support browsers by adjusting the `browserslist` key in `package.json` accoring to the [Browserslist specification](https://github.com/browserslist/browserslist#readme).
+
 For example, this:
 
 ```css