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.
Agreed. I think there also needs to be a boundary after which an RFC is needed. 

E.g. "Remove rest-client Dependency" shouldn't require an RFC where as "Add DSC Resource" should.