gitmodules.5: url can be a relative path

There is already excellent documentation for this facility in
git-submodule.1, but it is not so discoverable.

Relative paths in .gitmodules can be useful for serving the
same repository over multiple protocols, for example.
Thanks to Peter for pointing this out.

Cc: Peter Krefting <peter@softwolves.pp.se>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitmodules.txt

@@ -29,6 +29,9 @@ submodule.<name>.path::
 
 submodule.<name>.url::
 	Defines an url from where the submodule repository can be cloned.
+	This may be either an absolute URL ready to be passed to
+	linkgit:git-clone[1] or (if it begins with ./ or ../) a location
+	relative to the superproject's origin repository.
 
 submodule.<name>.update::
 	Defines what to do when the submodule is updated by the superproject.