title: Combining field values with redux-form v6
body: <p>I'm migrating part of an application from <code>redux-form v5</code> to <code>v6</code> (<code>alpha-10</code>). I need to display the combined values of some fields. </p>
  
  <p>In <code>v5</code>, I would have something like:</p>
  
  <pre><code>class MyForm extends Component {                            
    render() {                                                
  
      const { fields: { first, last }, handleSubmit } = this.props
  
      return (                                                
        &lt;form onSubmit={handleSubmit}&gt;                        
  
          &lt;div&gt;                                               
            &lt;label&gt;First name&lt;/label&gt;
            &lt;input type="text" {...first}/&gt;
          &lt;/div&gt;                                              
  
          &lt;div&gt;                                               
            &lt;label&gt;Last name&lt;/label&gt;
            &lt;input type="text" {...last}/&gt;
          &lt;/div&gt;
  
          &lt;div&gt;
            Welcome {first.value last.value.toUpperCase()}
          &lt;/div&gt;
  
          &lt;button type="submit"&gt;Submit&lt;/button&gt;               
        &lt;/form&gt;                                               
      )
    }
  }
  </code></pre>
  
  <p>The last <code>div</code> of the form combines the values of my two <code>first</code> and <code>last</code> fields. How can I create such an element with <code>v6</code> ?</p>