[PATCH] Documentation for io-accounting / reporting via procfs

Add some documentation for the new and very useful io-accounting feature.
It's being added to Documentation/filesystems/proc.txt

Signed-off-by: Roland Kletzing <devzero@web.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>