doc: Add running-weston.rst file

Documentation file for explaning in more detail how to run weston, using launcher direct and specifying a non-root seat. This explains how to create a udev file and assign a particular GPU card to it, and potentially, other input devices. It also describes a bit additional arguments that can be passed on, as an introduction to using the DRM node for that particular seat.

Fixes #460 (closed)

Signed-off-by: Marius Vlad marius.vlad@collabora.com