NIP-39:
https://github.com/nostr-protocol/nips/commit/354c93aea363b053d9a91371cc62386b7e4e7302
Human readable form here, it just hasn't been added to readme.md yet:
https://github.com/nostr-protocol/nips/blob/master/39.md
https://github.com/nostr-protocol/nips/blob/master/39.md
🤙2