tar-tree: illustrate an obscure feature better

Since you can tar just a subdirectory of a certain revision, tell
the users so, by showing an example how to do it.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-tar-tree.txt

diff --git a/Documentation/git-tar-tree.txt b/Documentation/git-tar-tree.txt
index 7a99acf..1e1c7fa 100644
--- a/Documentation/git-tar-tree.txt
+++ b/Documentation/git-tar-tree.txt
@@ -71,6 +71,11 @@
 
 	Get a tarball v1.4.0 from example.com.
 
+git tar-tree HEAD:Documentation/ git-docs > git-1.4.0-docs.tar::
+
+	Put everything in the current head's Documentation/ directory
+	into 'git-1.4.0-docs.tar', with the prefix 'git-docs/'.
+
 Author
 ------
 Written by Rene Scharfe.