Add note when using stateless root component in 1.x
Add note when using stateless root component in 1.x

When the root component is written as stateless plan function, instead `React.Component`, the modules won't reload and you always get the `The following modules couldn't be hot updated: (They would need a full reload!)` warning.
This can be easily replicated changing the App.js of the [boilerplate](https://github.com/gaearon/react-hot-boilerplate) into:
```javascript
import React, { Component } from 'react';

export default function App() {
  return (
    <h1>Hello, world.</h1>
  );
}
```

I don't know if it is already fixed in the new version or if it was already known as problem, but I lost some hour before discovering it on my own so I think it would help adding it to the docs to save someone else's time 😬
(My root component was written as pure function because of ESLint with Airbnb's config, which warns about writing stateless components as classes.)

gaearon/react-hot-loader,gaearon/react-hot-loader