Add explicit notes regarding Rails 4.

The recent yanking of version 2.0.alpha.0 has caused some confusion about compass-rails due to not updating the version to 2.0. There's also no explicit mention of Rails 4 in the Readme which lead me to question why I couldn't get compass-rails working although it was successfully installed. I also suffered as I was using the forked version who's respository was removed without a note on what to replace it with. I think some notes along these lines should hopefully help others who have had the same confusion as myself.