Add a Rails 4 compatibility note to the README.

We're using `paper_trail` on an application recently upgraded to Rails 4.

Once we completed the migration to strong parameters and removed the `protected_attributes` gem, we got a `MassAssignmentSecurity::Error` because of an `attr_accessible` in a `paper_trail` initializer which monkey-patches the `PaperTrail::Version` class.

`attr_accessible` shouldn't be required in a Rails 4 app that doesn't use `protected_attributes`, and this PR updates the README to reflect this.