message: Add root private key for wallet add
diff: @@ -131,6 +131,13 @@ Wallets.prototype.add = function(params, callback) {
  walletParams.rootPub = params.rootPub;
  }
  + // In XLM this private key is used only for wallet creation purposes, once the wallet is initialized then we
  + // update its weight to 0 making it an invalid key.
  + // https://www.stellar.org/developers/guides/concepts/multi-sig.html#additional-signing-keys
  + if (params.rootPrivateKey) {
  + walletParams.rootPrivateKey = params.rootPrivateKey;
  + }
  +
  if (params.initializationTxs) {
  walletParams.initializationTxs = params.initializationTxs;
  }
  