Add documentation for Multiboot

The documentation shows how to use -kernel and friends for booting Linux,
but obviously knows nothing about multiboot yet.

Let's include some documentation for multiboot, so people know how to fully
exploit this cool new feature.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
