Update README to clarify usage of :invalid_resource / RocketPants::Inval...

After spending hours the other day on digging into the way Rocket Pants handles `:invalid_resource`, I finally figured out that it needs an extra argument (from this line: https://github.com/filtersquad/rocket_pants/blob/v1.7.0/lib/rocket_pants/errors.rb#L70 )

I hope this clarifies how to use `:invalid_resource` properly. :+1: