Generated CSS is not exported as default as readme says

According to the [code](https://github.com/egoist/rollup-plugin-postcss/blob/71593d9f4698ce564482b86cc2fa7f69626e8b8a/src/postcss-loader.js#L207), when `extract` is false, the generated stylesheet is exported as `stylesheet`, not what current doc says:

```
...and the CSS string is also available as default export unless extract: true:
```

It should be something like:

```js
// Inject to `` and also available as `styleSheet`
import style, { stylesheet } from './style.css'
```