Add usage documentation

[jpmckinney]

At present I don’t know what to do with this repository’s output.

(Also, when switching fully to ocdsextensionregistry in a recent commit, some extension rows were re-ordered in the output. I don’t know if this matters, since I don’t know how the output is used.)

[yolile]

As part of this, we should prepare a full process note on how to update/create mapping templates see https://crm.open-contracting.org/issues/7549#note-35 for context