Accessibility considerations (#146)

Add section reminding implementors to follow OS level accessibility
guidelines.

resolves #145

Author: ewilligers

index.html

@@ -363,6 +363,16 @@ <h3>
         </p>
       </section>
     </section>
+    <section class="informative">
+      <h2>
+        Accessibility considerations
+      </h2>
+      <p>
+        When this specification is used to present information in the user
+        interface, implementors will want to follow the OS level accessibility
+        guidelines for the platform.
+      </p>
+    </section>
     <section class="informative">
       <h2>
         Security and privacy considerations