Merge pull request bitcoin#1331 from ShenghaiWang/master

Add one Swift implementation

Author: luke-jr

bip-0039.mediawiki

@@ -188,6 +188,7 @@ Swift:
 * https://github.com/pengpengliu/BIP39
 * https://github.com/matter-labs/web3swift/blob/develop/Sources/web3swift/KeystoreManager/BIP39.swift
 * https://github.com/zcash-hackworks/MnemonicSwift
+* https://github.com/ShenghaiWang/BIP39
 
 C++:
 * https://github.com/libbitcoin/libbitcoin-system/blob/master/include/bitcoin/system/wallet/mnemonic.hpp