Introduce skeleton SUPPORT.md

Introduce skeleton SUPPORT.md

Add a machine-readable file to describe what features are in what
state of being 'supported', as well as information about how long this
release will be supported, and so on.

The document should be formatted using "semantic newlines" [1], to make
changes easier.

Begin with the basic framework.

Signed-off-by: Ian Jackson <ian.jackson@citrix.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
[1] http://rhodesmill.org/brandon/2012/one-sentence-per-line/