tutorial: setting up a tree for subsystem maintainers

The "copying over packs" step is to prevent the objects
available in upstream repository to get expanted in the
subsystem maintainer tree, and is still valid if the upstream
repository do not live on the same machine.  But if they are on
the same machine using objects/info/alternates is cleaner.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/tutorial.txt

@@ -1534,7 +1534,10 @@ on that project and has an own "public repository" goes like this:
    the "project lead" person does.
 
 3. Copy over the packed files from "project lead" public
-   repository to your public repository.
+   repository to your public repository, unless the "project
+   lead" repository lives on the same machine as yours.  In the
+   latter case, you can use `objects/info/alternates` file to
+   point at the repository you are borrowing from.
 
 4. Push into the public repository from your primary
    repository. Run `git repack`, and possibly `git prune` if the