Add description for LXC template usage

Document how to use template for LXC container creation capable to boot
into OStree deployment.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>
Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D7319