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>

Author: phil-blain

Documentation/gitmodules.txt

@@ -121,7 +121,7 @@ submodules a URL is specified which can be used for cloning the submodules.
 
 SEE ALSO
 --------
-linkgit:git-submodule[1] linkgit:git-config[1]
+linkgit:git-submodule[1], linkgit:gitsubmodules[7], linkgit:git-config[1]
 
 GIT
 ---