Issue note-two-sses

Allowing unqualified references to the built-in simple types seems like
a really bad idea to me.  It makes it harder for the non-expert who
hasn't memorised the set of built-in simple datatypes to understand
schemas: when they see <element name="orderDate" type="date"/>, they
cannot tell whether they should look in the schema for a definition of
date or whether they should look in the Datatypes spec for a built-in
definition.  It perhaps makes schemas easier to write, but it makes them
harder to read, and that's a poor trade-off: schemas will be read more
often than they are written.

James