Document git-ls-files --directory

Add the appropriate bit of documentation.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-ls-files.txt

@@ -46,6 +46,10 @@ OPTIONS
 -s|--stage::
 	Show stage files in the output
 
+--directory::
+	If a whole directory is classified as "other", show just its
+	name (with a trailing slash) and not its whole contents.
+
 -u|--unmerged::
 	Show unmerged files in the output (forces --stage)