Add author attribution to ndistinct README

[samimseih]

Description of changes: Adding missing attribution to the ndistinct README

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[jkatz]

Have we done this for other examples?

[adamguo0]

We haven't -- it's not too relevant for our existing examples since to my knowledge they weren't proposed or discussed elsewhere first (though I could be wrong about uuid_v7).

[jkatz]

If we go this route, I think for UUIDv7 we should reference the upstream PostgreSQL thread as this predates the implementation and did inspire our discussion around adding it: https://www.postgresql.org/message-id/CAAhFRxitJv%3DyoGnXUgeLB_O%2BM7J2BJAmb5jqAT9gZ3bij3uLDA%40mail.gmail.com

[adamguo0]

Makes sense, I can add that to this PR