Improve docs for hidden sources

[dylanratcliffe]

Currently hidden sources are only documented in a comment:

// Hidden Reports whether the items returned by this source should be marked as
// hidden in their metadata. Hidden items won't be shown in GUIs or stored in
// the database. This method is optional and since most sources aren't going to
// be hidden, you usually don't need it and it can simply be removed.
//
// Hidden sources will also be excluded from requests involving wildcards
//
// Uncomment this if you need the source to be hidden, if you're not sure, just
// delete this whole comment section
//
// func (s *ColourNameSource) Hidden() bool {
//  return false
// }

This should probably be better, like in the README