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 only want to remember that [Ruby 1.9.2 passed its EOL](https://www.ruby-lang.org/en/news/2014/07/01/eol-for-1-8-7-and-1-9-2/). I understand that what we are discussing here is the support of 1.9.3.