Incomplete documentation on creating web components

This issue was originally reported by @LukasBombach via facebook/react/issues/10155

Text of original issue:

your documentation on creating web components with react is quite cool and works really nice for the html. However, any component created this way will be unstyled as the documentation does not cover how to add CSS to the web component. Using create-react-app all your CSS will end up in a main.hash.css file and have selectors for your components, but when you create a web component, you css must be encapsulated in the component as the the components DOM lives in the shadow root and is not stylable from outside.

It would be cool if you could show a way to account for this.

Beginne mit dem Lesen der verlinkten Dokumentation zu Web Components und prüfe, wie die aktuelle React-Dokumentation dieses Thema strukturiert. Aktualisiere die Anleitung, um zu erklären, wie CSS für in einer Shadow Root gerenderte Komponenten gekapselt werden kann, mit einem Beispiel, das das von create-react-app generierte Stylesheet berücksichtigt; die Dokumentation sollte sowohl Markup als auch Styling klar abdecken.