Add a reference about muting canvas

Author: ericwbailey

partials/section.embedded.hbs

@@ -162,5 +162,10 @@
         height="400">
         Your browser does not support the <code>&lt;canvas&gt;</code> element.
         Please <a rel="external noopener" href="http://browsehappy.com/">consider upgrading your browser</a>.
-    </canvas><!-- End of #subsection__canvas -->
+    </canvas
+
+    <h3>Notes:</h3>
+    <p>
+        <small>Add <code>role="presentation"</code> to <code>&lt;track&gt;</code> to <a rel="external noopener" href="https://twitter.com/stevefaulkner/status/1039470607719694336">mute epeated announcement of graphic role by some screen readers</a>.
+    </p>><!-- End of #subsection__canvas -->
 </section><!-- End of #section__embedded -->