Adds documentation for the slub ABI.

This is placed in the `testing' directory since the meanings of these
files are still subject to change as slub is developed.
Acked-by: NChristoph Lameter <cl@linux-foundation.org>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NDavid Rientjes <rientjes@google.com>
Signed-off-by: NPekka Enberg <penberg@cs.helsinki.fi>