write overview of new citation fields

[mbjones]

New citation fields should be summarized in the spec, with examples of their use. These are:

• referencePublication
• usageCitation
• literatureCited

[srearl]

Hi @mbjones - Have a look at the eml-literature.md file that I added in a recent commit to see if I am on the right track documenting the citation types. You had already written a lot of the relevant text in other areas so much of it is recycled. I am struggling a bit with the subject-verb agreement as I think that literatureCited is a CitationListType where as usageCitation and referencePublication are CitationTypes (singular) but am not certain. Also, I have called this the eml-literature module in keeping with EML 2.1, but is this still a module?

[mbjones]

Thanks @srearl -- I'll take a look tomorrow on my flight to Europe. As you might have noticed, I started reorganizing into a set of markdown files (see #321), but didn't finish. But the materials are all there and building as a book now. I will continue reorganizing and will incorporate your literature material.

[mbjones]

Citation descriptions were integrated into the eml-modules-resources.md.