Docs Request: Show how to create a multi-column primary key

[DeflateAwning]

I think the way you define a multi-column primary key is by setting the primary_key=True argument on many of the columns. Clarifying this in the docs would be great though!

[AndreasAlbertQC]

Thanks for the suggestion @DeflateAwning ! See #157, hope this helps :)

[DeflateAwning]

This looks way better, thank you!!

Link for anyone lurking/reviewing: https://dataframely.readthedocs.io/en/latest/sites/features/primary-keys.html