Add mention of Tree being Focusable (#11008)

all/src/main/templates/release-notes.html

@@ -86,6 +86,7 @@
 
         <li><tt>Upload</tt> component now supports <tt>accept</tt> attribute.</li>
         <li><tt>SerializablePredicate</tt> has new helper APIs.</li>
+        <li><tt>Tree</tt> component now implements <tt>Focusable</tt></li>
 
         </ul>