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.
Maybe ask @sersut to weigh in on this argument because he previously stated [the opposite desire](https://github.com/opscode/chef-rfc/pull/21#discussion_r15376679). A middle ground would simply for me to write a column that says "tested on" and indicate where the tier 1 build is to be tested.