Note about ORM compatibility for association helper in the README and source

Hi,

I tried to use the association helper with Mongoid and got weird exceptions.
I had to search on the Web for half an hour before discovering that the association helper is currently not compatible with Mongoid :(.
As Devise, which is another gem from plataformatec, is compatible with Mongoid, I thought that it was the same for SimpleForm. Since it is not the same, I think there should be a note about that in the README.

Thanks.

Nicolas.