resolves #22; re. risks of evolving schema.org vocab

[deiu]

This PR fixes #22. I've added some text mentioning the use of schema.org and its mutable nature. Are you OK with this text, @TallTed?

I would like to avoid starting a philosophical debate regarding vocabulary immutability in RDF, and focus instead on just the caveats of schema.org. I believe @dlongley covered it sufficiently in the paragraph above the one I just added.

If we are going to use someone else's vocabulary, we will want to make sure it
is stable and unlikely to change in any significant way. There may even be
technologies that we can make use of that store immutable vocabularies that
we can reference, but those are not the focus of this example.

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[TallTed]

Please note that my points are not philosophical. The text discusses the importance of using an un-changing vocab for VCs, and then uses a changing vocab for the example. The example would be much better (and more realistic) if it used versioned URIs for all schema.org terms, or switched to using an un-changing (i.e., already and more cleanly versioned) vocab.