Updates

It would be particularly helpful to review the README and make sure it makes sense.
Hm, I haven't seen any relevant code in the Rails source, but I'm going to keep this as is. I think the term `record` makes more sense for an ActiveRecord instance, and `model` is a good term for the model object, which is the class that a record is an instance of. `model_class` almost seems more like the parent class of the model object to me.