LXC appliance improve docs to make it easier to get started

[JedMeister]

Via assisting a new TurnKey and LXC user in the forums it's become apparent that our current documentation (inc the appliance page) is sub-optimal, especially for users new to LXC and/or TurnKey.

Out of the discussions there, I have 3 changes that we should implement:

1. comment right at the top of the template script to explicitly note that it should be called via
   lxc-create -t turnkey <other-lxc-options> -- <turnkey-template-options> rather than directly.
2. move the link to the usage docs much higher on the doc page (and link directly to the usage doc itself, rather than the docs directory). I.e. edit the readme (the appliance page is generated from the readme).
3. include an example command (using lxc-create - perhaps the one noted in point 1) on the main appliance page (or at least a note about how to leverage the template as a dot point under the info about the turnkey template). Again adjust the readme (as per point 2).

[JedMeister]

As per #1520 LXC has been deferred for now...