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

lxc/readme.md

+LXC template generates configuration file for container, pre-mount hook and AppArmour profile.
+Template is compatible with LXC upstream, so tools from LXC should be used to create/start/stop/destroy the container.
+
+# Create container:
+ sudo lxc-create -t $PWD/lxc-apertis-ostree -P $PWD/apertis --name apertis-test -- --ospack "https://images.apertis.org/daily/17.12/images/minimal/arm64-generic/20171205.0/ospack_17.12-amd64-minimal-lxc_00000000.0.tar.gz"
+
+options:
+ - -t -- use template named `lxc-apertis-ostree` from current directory
+ - -P -- Use an alternate container path. The default is /var/lib/lxc
+ - --name -- name of container
+ - -- separator. Options for template must be added after this separator
+ - --ospack -- URL to download the initial image
+
+# Start the container in foreground mode:
+ sudo lxc-start -F -P $PWD/apertis --name apertis-test
+
+Pull the ostree, deploy and reboot into deployed OS tree:
+
+ sudo ostree admin upgrade -r
+
+# Destroy the container and associated configuration:
+ sudo lxc-destroy -P $PWD/apertis --name apertis-test