Update RAC starter README to include how to support CSS Nesting in prouction (#5746)

Author: reidbarber

starters/docs/README.md

@@ -8,3 +8,7 @@ To get started, run the following commands:
 yarn
 yarn storybook
 ```
+
+## Building for Production
+
+This starter uses [CSS Nesting](https://drafts.csswg.org/css-nesting/), which is supported in the latest version of all major browsers, but if further support is needed, you can compile this to flattened selectors by enabling the feature in [Lightning CSS](https://lightningcss.dev/docs.html) or using the [PostCSS Nesting](https://github.com/csstools/postcss-plugins/tree/main/plugins/postcss-nesting#usage) plugin in your build.