add GITenberg-style metadata.yaml to each repo

[JonathanReeve]

See https://github.com/GITenberg/Adventures-of-Huckleberry-Finn_76/blob/ef16d1efe4018854829ecd7be0da87f1a73ee881/metadata.yaml for an example.

[JonathanReeve]

Spec available here: https://github.com/gitenberg-dev/documentation/blob/master/metadata/pandata_attribute_dictionary.yaml

[tfmorris]

Happy to use YAML for metadata and the GITenberg spec could be used as a starting point, but would need to be reviewed and revised first. The choice of different keys for lists vs single values is weird, so who knows what else will be found on closer inspection.

[lb42]

Gutenberg provides all this information in RDF, which is a bit easier to handle with generic XSLT tools.

[tfmorris]

RDF has many serializations, only one of which is XML. XSLT was super cool in the '90s. Now, maybe not so much.