Document Linux Memory Policy

I couldn't find any memory policy documentation in the Documentation
directory, so here is my attempt to document it.

There's lots more that could be written about the internal design--including
data structures, functions, etc.  However, if you agree that this is better
that the nothing that exists now, perhaps it could be merged.  This will
provide a baseline for updates to document the many policy patches that are
currently being worked.

Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Christoph Lameter <clameter@sgi.com>
Cc: Andi Kleen <ak@suse.de>
Cc: Michael Kerrisk <mtk-manpages@gmx.net>
Acked-by: Rob Landley <rob@landley.net>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>