gitmodules: link to gitsubmodules guide

gitmodules: link to gitsubmodules guide

Presently in the manpages git-submodule[1] links to gitsubmodules[7]
and gitmodules[5], gitsubmodules[7] links to git-submodule[1] and gitmodules[5],
but gitmodules[5] only link to git-submodule[1] (and git-config[1]).

Add a link to gitsubmodules[7] in gitmodules[5], so that a person
stumbling upon gitmodules[5] can quickly access gitsubmodules[7],
which has a more high-level overview of submodule usage.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>