Updated the primer

FWIW, I have made a small update of the primer[1] after editing it yesterday and today. Apart from stylistic changes, the two bigger ones are:

- I have exchanged the reference to SIOC to schema.org (I think having a reference to schema.org is a must and it fitted nicely)
- I have also added a small subsection on the fact that @property may take a list of values

I am close to declaring it 'done' as far as I am concerned... though I still have to adapt the example file.

Ivan


[1] http://www.w3.org/2010/02/rdfa/sources/rdfa-primer/Overview-src.html

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Ivan Herman, W3C Semantic Web Activity Lead
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