document alternative random container names

Perhaps we'll decide not to do it this way, but let's add the
possibility to the spec and consider it.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Merge pull request #16 from stgraber/master

Tweak README a little bit
Merge pull request #15 from hallyn/m.2

document alternative random container names