Add documentation about empty functional components

docs/05-reusable-components.md

@@ -299,4 +299,8 @@ However, you may still specify `.propTypes` and `.defaultProps` by setting them
 >
 > Because stateless functions don't have a backing instance, you can't attach a ref to a stateless function component. Normally this isn't an issue, since stateless functions do not provide an imperative API. Without an imperative API, there isn't much you could do with an instance anyway. However, if a user wants to find the DOM node of a stateless function component, they must wrap the component in a stateful component (eg. ES6 class component) and attach the ref to the stateful wrapper component.
 
+> NOTE:
+>
+> Due to some implementation details, returning `null` or `false` to indicate you don't want to render anything won't work in a functional component (see [react/4599](https://github.com/facebook/react/issues/4599) for details). Consider returning `<noscript />` instead.
+
 In an ideal world, most of your components would be stateless functions because in the future we’ll also be able to make performance optimizations specific to these components by avoiding unnecessary checks and memory allocations. This is the recommended pattern, when possible.