Please clearify possible association types on docs

[chmich]

THANKS for this easy and mighty gem!

Similar to paper_trail-association_tracking, could you please add a little sentence, which association types are possible to track?

Are polymorphic associations possible?

Example:
On the docu you describe a has_many association. That works fine. I made a test, to the other direction: the belongs_to association didn't work. Of course, that makes no sense in most cases, but it would be helpful to know which types are supported.

Thanks,
Chris