manual: add virtual package tutorial

The manual now features a new section with instructions about how to add a
virtual package.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
[yann.morin.1998@free.fr: move down statement about provider's .mk]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>