Create documentation on registering a data source

[kzollove]

Data sources must be registered in the Postgres backbone.data_source table to be used within this framework. Registering can be a confusing, complex task depending on the complexity of the source dataset.

This documentation should probably be created as a vignette in the R package.

This documentation should give a thorough explanation of each column in the data source (geom_spec in particular) and walk a potential user through the process of creating a row in the table, updating the DDL for the backbone schema, and creating a PR for the new DDL.

Depends on

• Create documentation for creation and interpretation of the geom_spec and attr_spec #276

[kzollove]

This introduces the need for a framework for handling new data sources/ features. While a later iteration of this work will likely have an API built around the database to facilitate data source creation and persistence, for now we should have a robust method for requesting and accepting new data sources and features.

[kzollove]

Make sure that there is a part of this documentation that addresses how to create data_sources for offline use (use of download_url is different)