Add CSS Grid Layout instructions

Author: Timer

Diff for: packages/react-scripts/template/README.md

@@ -626,6 +626,10 @@ becomes this:
 
 If you need to disable autoprefixing for some reason, [follow this section](https://github.com/postcss/autoprefixer#disabling).
 
+[CSS Grid Layout](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Grid_Layout) prefixing is disabled by default, but it will **not** strip manual prefixing.
+If you'd like to opt-in to CSS Grid prefixing, [first familiarize yourself about its limitations](https://github.com/postcss/autoprefixer#does-autoprefixer-polyfill-grid-layout-for-ie).<br>
+To enable CSS Grid prefixing, add `/* autoprefixer grid: on */` to the top of your CSS file.
+
 ## Adding Images, Fonts, and Files
 
 With Webpack, using static assets like images and fonts works similarly to CSS.