Support (or document non-support) for Fragments

https://github.com/reactjs/reactjs.org/pull/345

React is adding a top-level React.Fragment export that transpiles to JSX as:

```jsx
<>
  <li>Stuff</li>
  <li>Things</li>
</>

React.createElement(React.Fragment, {}, ...)
```

Would Preact be able to support fragments or provide some type of fallback?