Documentation/git-submodule.txt: Add Description section

Documentation/git-submodule.txt: Add Description section

Figuring out how submodules work conceptually is quite a bumpy
ride for a newcomer; the user manual helps (if one knows to actually
look into it), but the reference documentation should provide good
quick intro as well. This patch attempts to do that, with suggestions
from Heikki Orsila.

Cc: Heikki Orsila <shdl@zakalwe.fi>
Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>