add react code example

Example 1: create react app

npx create-react-app my-app
cd my-app
npm start

Example 2: link react to html

<script crossorigin src="https://unpkg.com/react@17/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@17/umd/react-dom.development.js"></script>