title: How add mandatory dropdown field in Touch UI
body: <p>I added "required" as "true" but it is not working. "required" as "true" only works for text field. </p>
  
  <p>As per below document, I do not see any option to add mandatory field from dropdown. 
  <a href="http://docs.adobe.com/docs/en/aem/6-0/author/assets/managing-assets-touch-ui/managing-asset-schema-forms.html" rel="nofollow">http://docs.adobe.com/docs/en/aem/6-0/author/assets/managing-assets-touch-ui/managing-asset-schema-forms.html</a></p>
  
  <p>How is it possible to achieve this?</p>