Update documentation about RDF generation

[tmprd]

I couldn't find any existing documentation or guidance on how the current RDF is generated. This should help others who want to improve the process.

[tmprd]

Updated with location of new unit tests recently merged in, so I think this is ready.

@dbooth-boston I see that a link to a parser in HAPI was added, however I don't recall seeing this used for generating the Turtle in the official spec. As far as I can tell, only this TurtleParser.java should be used in the official publishing process for R5+, whereas this older RdfParser.java RDFParser.java was used in previous versions. Does the official FHIR spec really depend on HAPI?

[dbooth-boston]

@ericprud Can you answer Tim's questions above, about RdfParser.java and the FHIR spec depending on HAPI?

[tmprd]

This is ready! I'm pretty certain that HAPI is not used for generating RDF in the FHIR spec, because this PR links to every bit of code that is actually used for publishing it. Also, HAPI is out-of-date anyway: #137