add documentation

documentation covering edge cases, default behaviours and limitations would be nice.

would prefer adding it to main circe docs than a separate microsite. if no objections, i can start making a PR for it.

Sure, that'd be great! And I agree that it could go on the main Circe site (there's precedent for this in circe-generic-extras and circe-optics, both of which are now in separate repos).