Document git-clone --reference

The new --reference flag introduced to git-clone in
GIT 1.3.0 was not documented but is rather handy.
So document it.

Also corrected a minor issue with the documentation for the
-s flag; the info/alternates file name was spelled wrong.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-clone.txt

@@ -10,6 +10,7 @@ SYNOPSIS
 --------
 [verse]
 'git-clone' [-l [-s]] [-q] [-n] [--bare] [-o <name>] [-u <upload-pack>]
+	  [--reference <repository>]
 	  <repository> [<directory>]
 
 DESCRIPTION
@@ -46,10 +47,18 @@ OPTIONS
 -s::
 	When the repository to clone is on the local machine,
 	instead of using hard links, automatically setup
-	.git/objects/info/alternatives to share the objects
+	.git/objects/info/alternates to share the objects
 	with the source repository.  The resulting repository
 	starts out without any object of its own.
 
+--reference <repository>::
+	If the reference repository is on the local machine
+	automatically setup .git/objects/info/alternates to
+	obtain objects from the reference repository.  Using
+	an already existing repository as an alternate will
+	require less objects to be copied from the repository
+	being cloned, reducing network and local storage costs.
+
 --quiet::
 -q::
 	Operate quietly.  This flag is passed to "rsync" and
@@ -112,6 +121,16 @@ $ git show-branch
 ------------
 
 
+Clone from upstream while borrowing from an existing local directory::
++
+------------
+$ git clone --reference my2.6 \
+	git://git.kernel.org/pub/scm/.../linux-2.7 \
+	my2.7
+$ cd my2.7
+------------
+
+
 Create a bare repository to publish your changes to the public::
 +
 ------------