[SKOS] minor editorial changes for Reference?

Hello,

I have three minor comments Alistair and Sean might want to take into account for the last version of the Reference:

- reference to the Primer: the Primer is HTML-linked from the Reference many times, but it does not appear in the References section. The location to access it is therefore much less visible for the reader...

- reference to the Use Cases and Requirements. For the moment this document is not linked at all from the Reference. That's ok now, but if we release it as a Group Note, we may want to get more value from it, and cite it explicitly. If just as something the advanced reader might be interested in ;-) 

- LCSH as linked data is still referred to as lcsh.info, and not id.loc.gov.

Cheers,

Antoine