Add note about optgroup support

https://twitter.com/mfairchild365/status/1158105361200689154

Author: ericwbailey

partials/section.forms.hbs

@@ -251,6 +251,11 @@
           <option value="schnauzer">Schnauzer</option>
         </select>
       </label>
+
+      <h4 id="notes-select">Notes:</h4>
+      <p>
+        <small>The <code>optgroup</code> element has <a rel="external noopener" href="https://a11ysupport.io/tests/tech__html__select">mixed support with assistive technology</a>.</small>
+      </p>
     </fieldset>
     <!-- End of #subsection-select -->