[PATCH] Initramfs docs update

New section on creating an external initramfs image using cpio (with
script), a warning about bad advice in the cpio man page, a bit of
debugging advice (hello world and rdinit=/bin/sh), and a few minor tweaks
to other parts of it.

Signed-off-by: Rob Landley <rob@landley.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>