Added documentation for the additional method, require_from_group

I needed to use the additional method, require_from_group, and in order to do so I had to dig into the code to work out what parameters to pass. So I thought it might be helpful if I then wrote it up for your docs.

I've not tested this as I've not taken the time to build the full WordPress install of the docs site... just so you know!

Thanks for your work on jQuery Validation!