Documentation: external YAML files for citations not documented as supported

[DavidAntliff]

As verified in #1960, external YAML files are supported via both the --bibliography option and via inline YAML "bibliography: " when --filter pandoc-citeproc is specified, provided the YAML file is correctly formatted, however this is not documented as a supported format at http://johnmacfarlane.net/pandoc/README.html in the citations extension section.

[njbart]

Pull request created: #2078.
I’d also suggest replacing biblio.bib in the second and third examples of http://johnmacfarlane.net/pandoc/demos.html, “24. Markdown citations” by biblio.json and biblio.yaml and adding these files, so users can see examples of these file formats.
Also, for consistency I think that CSL YAML files used as biblio files (as opposed to being used as input files, but then these would be text files with a metadata section anyway) should not be required to start with references, see jgm/pandoc-citeproc#124.

[jgm]

+++ nickbart1980 [Apr 13 15 03:49 ]:

I’d also suggest replacing biblio.bib in the second and third examples of http://johnmacfarlane.net/pandoc/demos.html, “24. Markdown citations” by biblio.json and biblio.yaml and adding these files, so users can see examples of these file formats.

I'll do this.