Chef 12 scoping and Supported Versions of Chef

This RFC codifies our policy as a community for the supported versions of Chef and supported versions of Ruby for Chef. 

In the light of this policy it also contains a scoping exercise for Chef 12.
Unfortunately, there's not a Ruby 2.1 installer for windows yet, I think they're waiting for some code that's in 2.2. I would be happy with a move to requiring 2.0+ in 12 and announcing a general policy of not supporting EOL versions of ruby in the future. I could live with supporting 1.9 until next February if there's a compelling reason.