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.

先閱讀連結的 Web Components 文件，並檢視目前的 React 文件如何組織這個主題。更新指南，說明如何為呈現於 shadow root 中的元件封裝 CSS，並提供一個考量 create-react-app 產生的樣式表的範例；文件應清楚涵蓋標記與樣式。