Doc update for sign() method (#1010)

* Add multi-signing example to the sign() method

Explains why the `signAs` option is compulsory for multi-signing scenarios.

docs/index.md

@@ -5403,6 +5403,8 @@ options | object | *Optional* Options that control the type of signature that wi
 *options.* signAs | [address](#address) | *Optional* The account that the signature should count for in multisigning.
 secret | secret string | *Optional* The secret of the account that is initiating the transaction. (This field cannot be used with keypair).
 
+Please note that when this method is used for multisigning, the `options` parameter is not *Optional* anymore. It will be compulsory. See the multisigning example in this section for more details. 
+
 ### Return Value
 
 This method returns an object with the following structure:
@@ -5429,6 +5431,93 @@ return api.sign(txJSON, secret); // or: api.sign(txJSON, keypair);
 }
 ```
 
+### Example (multisigning)
+
+```javascript
+const RippleAPI = require('ripple-lib').RippleAPI;
+
+// jon's address will have a multi-signing setup with a quorum of 2
+const jon = {
+    account: 'rJKpme4m2zBQceBuU89d7vLMzgoUw2Ptj',
+    secret: 'sh4Va7b1wQof8knHFV2sxwX12fSgK'
+};
+const aya = {
+    account: 'rnrPdBjs98fFFfmRpL6hM7exT788SWQPFN',
+    secret: 'snaMuMrXeVc2Vd4NYvHofeGNjgYoe'
+};
+const bran = {
+    account: 'rJ93RLnT1t5A8fCr7HTScw7WtfKJMRXodH',
+    secret: 'shQtQ8Um5MS218yvEU3Ehy1eZQKqH'
+};
+
+// Setup the signers list with a quorum of 2
+const multiSignSetupTransaction = {
+    "Flags": 0,
+    "TransactionType": "SignerListSet",
+    "Account": "rJKpme4m2zBQceBuU89d7vLMzgoUw2Ptj",
+    "Fee": "120",
+    "SignerQuorum": 2,
+    "SignerEntries": [
+        {
+            "SignerEntry": {
+                "Account": "rnrPdBjs98fFFfmRpL6hM7exT788SWQPFN",
+                "SignerWeight": 2
+            }
+        },
+        {
+            "SignerEntry": {
+                "Account": "rJ93RLnT1t5A8fCr7HTScw7WtfKJMRXodH",
+                "SignerWeight": 1
+            }
+        },
+    ]
+};
+
+// a transaction which requires multi signing
+const multiSignPaymentTransaction = {
+    TransactionType: 'Payment',
+    Account: 'rJKpme4m2zBQceBuU89d7vLMzgoUw2Ptj',
+    Destination: 'rJ93RLnT1t5A8fCr7HTScw7WtfKJMRXodH',
+    Amount: '88000000'
+};
+
+const api = new RippleAPI({
+    server: 'wss://s.altnet.rippletest.net:51233'
+});
+
+api.connect().then(() => {
+    // adding the multi signing feature to jon's account
+    api.prepareTransaction(multiSignSetupTransaction).then((prepared) => {
+        console.log(prepared);
+        jonSign = api.sign(prepared.txJSON, jon.secret).signedTransaction;
+        api.submit(jonSign).then( response => {
+            console.log(response.resultCode, response.resultMessage);
+
+            // multi sign a transaction
+            api.prepareTransaction(multiSignPaymentTransaction).then(prepared => {
+                console.log(prepared);
+
+                // Aya and Bran sign it too but with 'signAs' set to their own account
+                let ayaSign = api.sign(prepared.txJSON, aya.secret, {'signAs': aya.account}).signedTransaction;
+                let branSign = api.sign(prepared.txJSON, bran.secret, {'signAs': bran.account}).signedTransaction;
+
+                // signatures are combined and submitted
+                let combinedTx = api.combine([ayaSign, branSign]);
+                api.submit(combinedTx.signedTransaction).then(response => {
+                    console.log(response.tx_json.hash);
+                    return api.disconnect();
+                }).catch(console.error);
+            }).catch(console.error);
+        }).catch(console.error)
+    }).catch(console.error);
+}).catch(console.error);
+```
+
+Assuming the multisigning account was setup properly, the above example will respond with `resultCode: 'tesSUCCESS'` and the hash for the transaction.
+If any of `{signAs: some_address}` options were missing the code will return a validation error as follow:
+```
+[ValidationError(txJSON is not the same for all signedTransactions)]
+```
 
 ## combine