Add reference to issue w3c#55.

specs/paymentrequest.html

@@ -796,6 +796,13 @@ <h2>PaymentResponse interface</h2>
       </dd>
       </dl>
 
+      <p class="issue" data-number="55" title="Should the API support field-level encryption?">
+There is a requirement for payment apps to be able to return data that is
+hidden from the payee themselves (perhaps for PCI scope reasons) as they will
+pass it on to their payment service processor who can then decrypt it and use
+it. This may be incorporated into the details object or elsewhere.
+      </p>
+
       <section>
         <h2>complete()</h2>
         <p>The <code><dfn>complete</dfn></code> method must be called after the user has accepted the payment