Updating the README with details about :strict

I was just updating a rails 3 app, and one of my tests was failing as it was expecting an `ActiveModel::MassAssignmentSecurity::Error` to be raised when a protected attribute was being sent through.

This just adds the `:strict` information to the README.

Thanks for the great work, now that the tests are all green I can go about removing this gem ... but this certainly made the transition process easier - so thanks very much! :+1: 
I believe it is better to show how configure it globally using `config.active_record.mass_assignment_sanitizer = :strict`