add create to the spec

I suspect we may actually want to support --ephemeral, but it's too
late at night to think through the cases of that (i.e. I assume we'd
only want persistent ephemeral containers, and in that case is it
worth having them).  So here's the basic blurb.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
The second part of that last sentence isn't relevant to the create command.