qmp-shell: add documentation

I should probably document the changes that were made.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <1435775149-17285-1-git-send-email-jsnow@redhat.com>
Reviewed-By: Kashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>