Merge pull request bitcoin#625 from yuzushioh/update-bip39

Add BIP39 Swift implementation link

Author: luke-jr

bip-0039.mediawiki

@@ -164,6 +164,7 @@ Rust:
 
 Swift:
 * https://github.com/CikeQiu/CKMnemonic
+* https://github.com/yuzushioh/WalletKit
 
 C++:
 * https://github.com/libbitcoin/libbitcoin/blob/master/include/bitcoin/bitcoin/wallet/mnemonic.hpp