Add some basic documentation on vfio-user usage. Signed-off-by: John Levon <john.levon@nutanix.com> Reviewed-by: Cédric Le Goater <clg@redhat.com> Link: https://lore.kernel.org/qemu-devel/20250625193012.2316242-19-john.levon@nutanix.com Signed-off-by: Cédric Le Goater <clg@redhat.com>