New Keyword section missing discussion of use of URL in text and diagram

[rduerr]

Looking at issue #125, it looks like guidance for when to use a URL for a keyword is missing from both the text and diagram in this section. The diagram needs to be updated (I apparently don't have access, so can't) and the text either needs to indicate the suggestion from that issue:

• use url for any non machine-readable URL for a term

or indicate that we recommend never using the URL and instead always using DefinedTerm.

I think we really meant the first of these. I can add that text but someone else needs to update the diagram!

[rduerr]

Actually it is hard for me to imagine a keyword with a URL that isn't from some sort of controlled vocabulary/glossary/dictionary; so that argues for recommending that folks always use DefinedTerm if you have a URL!