Docs: mention browserslist usage via browserslistrc file (#13810)

See docs https://github.com/browserslist/browserslist#readme

packages/browserslist-config/README.md

@@ -20,6 +20,12 @@ Add this to your `package.json` file:
 ]
 ```
 
+Alternatively, add this to `.browserslistrc` file:
+
+```
+extends @wordpress/browserslist-config
+```
+
 This package when imported returns an array of supported browsers, for more configuration examples including Autoprefixer, Babel, ESLint, PostCSS, and stylelint see the [Browserslist examples](https://github.com/ai/browserslist-example#browserslist-example) repo.
 
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