Update Concepts with OB key handling overview

[placer14]

A discussion in chat has produced the following overview of key handling in OB. We should incorporate this into the Concepts section with some additional fleshing-out.

As it currently stands, we do this:

ipfsSeed := bip39.NewSeed(mnemonic, "Secret Passphrase")

identityKey := IdentityKeyFromSeed(ipfsSeed) // this does the hmac with "OpenBazaar Seed". This is the ipfs key.

//////////////////////////////

bitcoinSeed := bip39.NewSeed(mnemonic, "")

masterPrivKey := bip32.NewMaster(bitcoinSeed)

pubkey := masterPrivKey.ECPublicKey()

pubkeyBytes := pubkey.SerializedCompressed() // this is what is in the 'bitcoin' field in the contract. Base64 serialized.

@cpacia notes that this would (hopefully) change in a future iteration of the software to allow for more flexible key handling.