How to contribute an extinction curve

[karllark]

Docs needed on how to contribute an extinction curve.

Requirements. E.g., need to be able to express it as A(lambda)/A(V).

An issue at minimum.

Ideally a PR with code, testing, docs. Detailed instructions needed. Previous PR as an example - #257.

Doc page already exists for code details. https://dust-extinction.readthedocs.io/en/latest/dust_extinction/dev_model.html