Add a YARD options file

[artob]

Please put this following in a .yardopts file, which will make it easy for me to use my existing tooling to re-generate the YARD documentation at http://rdf.rubyforge.org/rdfa/ after each rdf-rdfa release:

--title "RDF::RDFa - RDFa Support for RDF.rb"
--output-dir doc/yard
--protected
--no-private
--hide-void-return
--markup rdoc
--readme README.rdoc
-
AUTHORS
VERSION

[gkellogg]

Done.