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.

首先阅读链接的 Web Components 文档，并查看当前 React 文档是如何组织这一主题的。更新指南，说明如何为渲染到 shadow root 中的组件封装 CSS，并提供一个考虑到 create-react-app 生成的样式表的示例；文档应清楚涵盖标记和样式。