Added section and paragraph on accessibility considerations (#802)

in light of previous discussion of:
#450

Author: ianbjacobs

index.html

@@ -5149,6 +5149,18 @@ 
         p>
       section>
     section>
+    <section class="informative">
+      <h2>
+        Accessibility Considerations
+      h2>
+      <p>
+        For the user-facing aspects of Payment Request API, implementations
+        integrate with platform accessibility APIs via form controls and other
+        input modalities. Furthermore to increase the intelligibility of total,
+        shipping addresses, and contact information, implementations format
+        data according to system conventions.
+      p>
+    section>
     <section id="dependencies">
       <h2>
         Dependencies