docs: instructions for automatic HMR in react

[b-zurg]

This MR is inspired by this great solution electron/forge#1164 (comment) for adding HMR. I thought it was appropriate to put it in the docs as many will encounter this but not everyone would be using react.

[mbret]

Just tried and it works as expected. Havent tried on a big app so hard to say if there are unexpected behavior on the long run. 👏

Do you see any pitfall using it only once on the renderer and wrapping the App module ?

import { hot } from "react-hot-loader";
import './index.css';
import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';

ReactDOM.render(<App />, document.getElementById('root'));

export default hot(module)(App)

[malept]

This is blocked by #11

[erickzhao]

Thanks for this contribution!