Add reference to issue #56.

specs/paymentrequest.html

@@ -426,6 +426,14 @@ <h2>show()</h2>
           payment request after the <code>show</code> method returns.
         </p>
 
+        <p class="issue" data-number="56" title="Should a website be able to provide a label for the 'Buy' or 'Checkout' button displayed in the payment app?">
+          It may help users understand what they are accepting if the web site
+          is able to label the "accept" button. For example, if a user is about
+          to "Buy" something, "Reserve" something, "Subscribe" to something,
+          etc. That said, this may create payment interface/experience issues
+          and accidentally lead to customers thinking they're performing
+          actions like a one-time purchase, when they are in fact signing up
+          for a subscription.
         <p>
           The <a><code>show</code></a> method MUST act as follows:
         </p>