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.
I think we should also either announce that we're going to drop support for 1.9 when it goes EOL or just drop it when 12.0 ships, so we're not stuck supporting an EOL ruby version.