add nip-06, generating keys from mnemonic codes.

nips/06.md

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+NIP-06
+======
+
+Key derivation from mnemonic
+----------------------------
+
+`draft` `optional` `author:fiatjaf`
+
+This combines [BIP39](https://bips.xyz/39) and a modified [BIP32](https://bips.xyz/32) into a scheme that allows the generation of a private key from a list of seed words.
+
+BIP32 is modified to use the key `Nostr seed` instead of `Bitcoin seed` in the HMAC function.
+
+This document specifies the creation of a single Nostr private key, but it can be expanded in the future if "HD Wallets" ever become a relevant use case.
+
+Steps for generating the private key:
+
+1. Follow the steps on BIP39 for generating the `mnemonic` and then the `seed` (512 bytes);
+2. Calculate `result = hmac-sha512(key = 'Nostr seed', data = seed)`;
+3. Use the first 32 bytes of `result` as the private key.