Update README with transaction signing example

packages/ripple-keypairs/examples/sign-transaction.js

@@ -0,0 +1,45 @@
+var Transaction = require('ripple-lib').Transaction;
+var keyPairFromSeed = require('../').keyPairFromSeed;
+
+var SIGNING_PREFIX = [0x53, 0x54, 0x58, 0x00];
+
+function prettyJSON(obj) {
+  return JSON.stringify(obj, undefined, 2);
+}
+
+function signingData(tx) {
+  return SIGNING_PREFIX.concat(tx.serialize().buffer);
+}
+
+function signTxJson(seed, json) {
+  var keyPair = keyPairFromSeed(seed);
+  var tx = Transaction.from_json(json);
+  var tx_json = tx.tx_json;
+
+  tx_json.SigningPubKey = keyPair.pubKeyHex();
+  tx_json.TxnSignature = keyPair.signHex(signingData(tx));
+
+  var serialized = tx.serialize();
+
+  var id = tx.hash('HASH_TX_ID', /* Uint256: */ false , /* pre: */ serialized);
+
+  return {
+    hash: id,
+    tx_blob: serialized.to_hex(),
+    tx_json: tx_json
+  };
+}
+
+var seed = 'sEd7t79mzn2dwy3vvpvRmaaLbLhvme6';
+var tx_json = {
+  Account: 'r9LqNeG6qHxjeUocjvVki2XR35weJ9mZgQ',
+  Amount: '1000',
+  Destination: 'rHb9CJAWyB4rj91VRWn96DkukG4bwdtyTh',
+  Fee: '10',
+  Flags: 2147483648,
+  Sequence: 1,
+  TransactionType: 'Payment',
+};
+
+console.log('unsigned', prettyJSON(tx_json));
+console.log('signed', prettyJSON(signTxJson(seed, tx_json)));