Fix typos
Small refactors based on feedback from Pull Request #835

* Put readonly_attributes under simple_form_ prefix. This will further reduce
  our polluting of the object mapper's namespace. This will also help ensure
  we don't conflict with future object mappers that maybe already implement
  readonly_attributes but in a different way than ActiveRecord.
* Updated the documentation on the object mapper modules so it is clear what
  someone needs to do to add support for a new object mapper.