libnvdimm/security: Add documentation for nvdimm security support

Add theory of operation for the security support that's going into
libnvdimm.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jing Lin <jing.lin@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>