Per recent discussions in core, commit the updates to the tier
document as posted to the developers list about three months ago.
Some comments from that time have not been addressed, but will be in
subsequent commits. I had hoped to get them all ironed out before,
but I've been too swamped.

If you have additional comments, please feel free to send them to me.

This recognizes that FreeBSD is increasingly being used in embedded
targets and starts to define what is expected of these embedded
targets at different tier levels, and recognizes that we may do
releases for them differently than we do for server/desktop.