title: I need to have some fields in ui-schema which do not exist in schema section
body: <p>I used to use <a href="http://github.com/jsonform/jsonform/wiki" rel="nofollow noreferrer">jsonform</a> in my project but now I migrate to react, So I use this library (<a href="http://github.com/mozilla-services/react-jsonschema-form" rel="nofollow noreferrer">react-jsonschema-form</a>). 
  in jsonform I can have some fieldsets in form section that weren't  in schema. like this : </p>
  
  <pre><code>{
    "schema":
      {
        "firstName": {
        "type": "string",
        "title": "Is JSON Form useful?",
        }
      },
    "form": [
       {
        "key": "firstName",
        "type": "text",
       }, 
       {
        "title" : "this is non-schema",
        "type": "fieldset"
       }
     ]
  }
  </code></pre>
  
  <p>you can test it at <a href="http://ulion.github.io/jsonform/playground/" rel="nofollow noreferrer">jsonschema playground</a>.
  please help me to do something like above code in <a href="http://github.com/mozilla-services/react-jsonschema-form" rel="nofollow noreferrer">react-jsonschema-form</a>. 
  how can I have some fields that aren't in schema but I want to show them in uiSchema?</p>
  
  <p>react-jsonschema-form has a <a href="http://mozilla-services.github.io/react-jsonschema-form/" rel="nofollow noreferrer">playground</a> too. You can find a field named 'date'. It adds in uiSchema but it doesn't exist in schema section. Also nothing shows for this field in the result. I don't know why they use it if it couldn't be there!!!!</p>
  
  <p>thanks.</p>