Adapt README: `:force_non_association_create` is rendered obsolete.

I hope. I will not remove it yet for backward 
compatibility reasons. 
Not sure if somebody actually needs an object to be
_not_ created on the association. Maybe for 
other ORM's?
Add `:form_name` parameter. Fixes #153.

This gives users the option to use a different
form parameter than the default `f`.

Usage:

    link_to_add_association 'add something', form, :people, :form_name => 'person_form'