Documentation about unaligned memory access

Here's a document I wrote after figuring out what unaligned memory access
is all about.  I've tried to cover the information I was looking for when
trying to learn about this, without producing a hopelessly detailed/complex
spew.  I hope it is useful to others.

Signed-off-by: Daniel Drake <dsd@gentoo.org>
Cc: Rob Landley <rob@landley.net>
Cc: "Randy.Dunlap" <rdunlap@xenotime.net>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Jan Engelhardt <jengelh@computergmbh.de>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: Kyle McMartin <kyle@mcmartin.ca>
Cc: Kyle Moffett <mrmacman_g4@mac.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>