index.js with HMR for create-react-app - https://medium.com/@sheepsteak/adding-hot-module-reloading-to-create-react-app-e053fadf569d#.2q2q1mvxs

index.js

// https://medium.com/@sheepsteak/adding-hot-module-reloading-to-create-react-app-e053fadf569d#.2q2q1mvxs
import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';
import './index.css';
const rootEl = document.getElementById('root');
ReactDOM.render(
  <App />,
  rootEl
);
if (module.hot) {
  module.hot.accept('./App', () => {
    const NextApp = require('./App').default;
    ReactDOM.render(
      <NextApp />,
      rootEl
    );
  }); 
}