Add shadow mention

[JoviDeCroock]

Relates to https://github.com/preactjs/preact/issues/3485

[mikerob215]

Hey @JoviDeCroock, thanks for adding this to the docs. Maybe this is better in a separate issue but the docs also don't currently mention the hydrate attribute. In light of this issue: #52, it might be worth adding a mention of that attribute and noting that it wont work when shadow is set to true.