Platform support policy proposal

There is a fair amount of confusion about the specific operating system platforms, platform versions, etc. that Chef Software, Inc. supports for its products. This RFC will clarify this by calling out the individual operating systems and their releases as explicitly supported or not.
It might be nice to add a verify subcommand into core chef that acts like `chef verify` in ChefDK or `*-ctl test` in the server packages. Although it will just call some RSpec under the covers it can ensure the environment is fully configured and make CI testing easier.