Fix document

### Description

See section [Webpack Plugin](https://github.com/gaearon/react-hot-loader#webpack-plugin) in README.md

```javascript
{
test: /\.js$/,
include: /node_modules\/react-dom/,
use: ['react-hot-loader/webpack']
}
```

### Expected behavior

Change property `include` to `/node_modules[\\\/]react-dom/`, the reason is you should compatible with unix and window.

### Actual behavior

Only compatible with unix