Merge pull request bitcoin#969 from pacu/patch-1

Added MnemonicSwift as Swift impl for BIP-39

Author: luke-jr

bip-0039.mediawiki

@@ -169,6 +169,7 @@ Swift:
 * https://github.com/CikeQiu/CKMnemonic
 * https://github.com/yuzushioh/WalletKit
 * https://github.com/matter-labs/web3swift/blob/develop/Sources/web3swift/KeystoreManager/BIP39.swift
+* https://github.com/zcash-hackworks/MnemonicSwift
 
 C++:
 * https://github.com/libbitcoin/libbitcoin-system/blob/master/include/bitcoin/system/wallet/mnemonic.hpp