Added note for DataSets table.

IEDA suggested, at 2013-12-17 meeting, that the DataSets table conform
to Dublin Core ISO metadata standard
(http://dublincore.org/documents/dces/) and that we look at
DataCite.org
(http://schema.datacite.org/meta/kernel-3/doc/DataCite-MetadataKernel_v3
.0.pdf).