Add readonly option to has_many

Hello! As i describe in the ticket description i find very useful
sometimes that a has_many relationship is readonly. For example, a
class may access a collection of objects only to read them, but can't
save them (i.e. reports or payments).

There is an existing option to ActiveRecord find method to do this,
but with this enhancement you may specificate that at a higher scope,
a feature that already exists for order, uniq, etc. The functionality
is tested and documented, i hope you find it as useful as me and would
love some feedback.