fix(guide): add callout to broken example

public/docs/js/latest/guide/user-input.jade

@@ -14,6 +14,12 @@
     As in previous examples, you can make element references available to other parts of the template as a local
     variable using the <code>#var</code> syntax.  With this and events, we can do the old "update text as you type" example:
 
+  .callout.is-helpful
+    header Broken Example
+    p
+      span This specific example is currently broken, pending resolution of&nbsp;
+      a(href="https://github.com/angular/angular/issues/2019") 2019
+
   code-example(language="html").
     &lt;input #myname (keyup)&gt;
     &lt;p&gt;{{myname.value}}&lt;/p&gt;