Primer updated with a Changes section

Hi all,

I just added [Section 6. Changes] to the Primer:

http://www.w3.org/2006/07/SWD/RDFa/primer/

which I wrote up based on the XML diff of the two Overview.xml files. 
Little has changed. If you have time, please review it today or 
tomorrow! Look for examples that I forgot to fix!

Thanks,

-Ben