added many-to-many-on-the-same-table relationship

I'm not sure how detailed the documentation should be, but it seems like useful information to me.
Also changed 'has_and_belongs_to_many' to 'has_many_and_belongs_to'.

Pull requests and contributing are kinda new to me, so tell me if I'm doing things wrong/can do things better.
Signed-off-by: H.J. van Meerveld <haaiee(a)gmail.com>