Added basic Nokogiri migration guide.
Added basic Nokogiri migration guide.

I will expand this over time, but it's a decent start for the time being.

This fixes #13.

YorickPeterse/oga,dfockler/oga,YorickPeterse/oga,altmetric/oga,YorickPeterse/oga,jeffreybaird/oga,dfockler/oga,altmetric/oga,jeffreybaird/oga,ttasanen/oga,YorickPeterse/oga,dfockler/oga,altmetric/oga,ttasanen/oga,ttasanen/oga,dfockler/oga,altmetric/oga,ttasanen/oga,ttasanen/oga,jeffreybaird/oga,altmetric/oga,jeffreybaird/oga,jeffreybaird/oga,YorickPeterse/oga,dfockler/oga