docs/manual: document meson-based packages

Add instructions for adding a package which uses the Meson build system.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>