docs: flat config example for react 17+

Author: GabenGar

README.md

@@ -221,6 +221,7 @@ const reactPlugin = require('eslint-plugin-react');
 module.exports = [
   …
   reactPlugin.configs.flat.recommended, // This is not a plugin object, but a shareable config object
+  reactPlugin.configs.flat["jsx-runtime"], // Add this if you are using React 17+
   …
 ];
 ```