Add documentation about the kmem controller

Signed-off-by: Glauber Costa <glommer@parallels.com>
CC: Frederic Weisbecker <fweisbec@redhat.com>
CC: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
CC: Michal Hocko <mhocko@suse.cz>
CC: Christoph Lameter <cl@linux.com>
CC: Pekka Enberg <penberg@cs.helsinki.fi>
CC: Johannes Weiner <hannes@cmpxchg.org>
CC: Suleiman Souhlal <suleiman@google.com>
CC: Tejun Heo <tj@kernel.org>