Update docs with shadow dom guide and element passing

[aziz512]

Hey maintainers! I recently found your library and I've liked it a lot. When first reading the docs it was very unclear as to whether I can use it with Shadow DOM without doing too much hackery. Upon inspection of the source I noticed that it's written very elegantly and does indeed permit my use case.

That being said, this PR adds docs as to how to use it with Shadow DOM or by passing a DOM element.

[kazzkiq]

Thanks for the addition to documentation!