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>