mongrel2: Add comments to avoid use of github call

As the comment says, we don't want to uses the github helper, as we
would end up with a generated tarball, which is not the same one as the
one linked with the releases.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>