add xDD as a data source

[mbjones]

The xDD project (originally GeoDeepDive, now broader) has mined a bunch of text sources to locate citations and references, and could be a good data source for scythe.

• API: https://xdd.wisc.edu/api

[theamarks]

To-Do List

• Meet with Jeanette - learn how to add #
• add xDD as a data source #32