Document .git/modules

A note in the beginning of this document describes the behavior already.
This patch just adds where to find the repositories.

Signed-off-by: Fredrik Gustafsson <iveqy@iveqy.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitrepository-layout.txt

@@ -211,6 +211,9 @@ shallow::
 	and maintained by shallow clone mechanism.  See `--depth`
 	option to linkgit:git-clone[1] and linkgit:git-fetch[1].
 
+modules::
+	Contains the git-repositories of the submodules.
+
 SEE ALSO
 --------
 linkgit:git-init[1],