manual: Add notes about GitHub and hashes

We can't take hashes from GitHub, unless the tarball has been uploaded by
the maintainer, otherwise it is generated and may change over time,
which renders hash files useless.

[Peter: slightly reword]
Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>