Platform support policy proposal

:construction: but you can comment on it. Mostly the construction is missing versions for OSes that I don't know that much about (Arch, Gentoo, etc.)
We have a similar lifecycle policy thing for ChefDK to what's in the appendix below, but is more aggressive about EOLing versions. I think it'd be a good idea to make that public, either via this RFC or a different one, and also revisit some of those decisions. The current policy for ChefDK on Mac is N, with N - 1 supported for a year (which we are not in compliance with). /cc @sersut