CSS in plugins

[asportnoy]

This could be a new page or a section on an existing page, but we should mention that plugins can include css by just importing the file. Should also mention something about choosing good class names to avoid conflicts and allow users to modify the styles with quick CSS.
Example: https://github.com/asportnoy/role-dividers/blob/a9e92b3887fe1ca660de421ead0e2b9fbbdad5a1/src/index.tsx#L10