updated README about contracts

It seems to me that `README` contains a small typo. Instead of asking a user to `update_attributes` before calling `validate` we should rather `assign` them (as `update_attributes` would silently change the basic model object without `contract`'s validations).
More then that, code sample below that part of `README` points to `assign_attributes` method aswell.

> Applying a contract is simple, all you need is a populated object (e.g. an album after #**update_attributes**).

>    album.**assign_attributes**(..)