Minimalist proposal to resolve no-primary-key issue

I think we just need to fix the DM.  If you disagree, please indicate what else needs to be said.

The DM spec says:
[[The Direct Mapping is intended to provide a default behavior for R2RML: RDB to RDF Mapping Language <http://www.w3.org/TR/2012/CR-r2rml-20120223/> [R2RML] <http://www.w3.org/TR/rdb-direct-mapping/#R2RML>. It can also be used to materialize RDF graphs or define virtual graphs, which can be queried by SPARQL or traversed by an RDF graph API.]]

Add an asterisk after the first sentence and a footnote.  The footnote says:
[[Except in the case of tables or views without a primary key.  In this case, identical rows may be kept distinct
by the DM and collapsed into a single row by R2RML]]

R2RML says:
[[This specification has a companion that defines a direct mapping from relational databases to RDF <http://www.w3.org/TR/rdb-direct-mapping/> [DM <http://www.w3.org/TR/r2rml/#DM>]. In the direct mapping of a database, the structure of the resulting RDF graph directly reflects the structure of the database, the target RDF vocabulary directly reflects the names of database schema elements, and neither structure nor target vocabulary can be changed. With R2RML on the other hand, a mapping author can define highly customized views over the relational data.]]

No change needed.
-- 
All the best, Ashok