documentation: begin documenting our use of git submodules

git submodules have some surprising behaviour, and we make full use
of it.  Start building a canonical resource for that so developers
can spend their time on better things than on fighting git.

Change-Id: I5aa721e9b0acb2912a057858fd23a1d59d845ed1
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/8371
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

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