thephpleague#261 declare transactionId

In conjunction with thephpleague#261 & thephpleague/omnipay-common#41

Author: eileenmcnaughton

README.md

@@ -326,6 +326,7 @@ $response = $gateway->purchase(['amount' => '10.00', 'card' => $card])->send();
 $response->isSuccessful(); // is the response successful?
 $response->isRedirect(); // is the response a redirect?
 $response->getTransactionReference(); // a reference generated by the payment gateway
+$response->getTransactionId(); // the reference set by the originating website if available.
 $response->getMessage(); // a message generated by the payment gateway
 ```