Mention 'cpio' dependency in INSTALL

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

INSTALL

@@ -79,6 +79,9 @@ Issues of note:
 	- "perl" and POSIX-compliant shells are needed to use most of
 	  the barebone Porcelainish scripts.
 
+	- "cpio" is used by git-merge for saving and restoring the index,
+	  and by git-clone when doing a local (possibly hardlinked) clone.
+
  - Some platform specific issues are dealt with Makefile rules,
    but depending on your specific installation, you may not
    have all the libraries/tools needed, or you may have