Document adding margin-bottom to body when peek is enabled

This is a followup to https://github.com/peek/peek/pull/52

If you have more content than can fit on a single page, peek would end up covering up the bottom-most content. This recommends making a body class to indicate peek's enabled, then when peek is present, applying that class.

It might be simpler to get peek's height and automatically add that to body though.