README: Update packaging section for Git repositories

Change-Id: I8d9c56be8c1381b175ce7695c53f31b1767d9d17
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/22454
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
(cherry picked from commit 6891709a1f04a78bc45ad4174f4416f24169a020)
Reviewed-on: https://review.coreboot.org/22672
1 file changed