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.

Start by reading the linked web components documentation and reviewing how the current React documentation organizes this topic. Update the guidance to explain how CSS can be encapsulated for components rendered in a shadow root, with an example that accounts for create-react-app's generated stylesheet; the documentation should clearly cover both markup and styling.