title: How to provide Picker a default "Please select..." option?
body: <p>I'd like to make my Picker to show a "default option" on startup. That means: something like "Please select an option".</p>
  
  <p>I've tried to add a option manually with this phrase, but, this way the "default option" can be re-selected after selecting other options, like it was one of the real options. There is some way to do this?</p>
  
  <pre><code>&lt;View style={Styles.row}&gt;
              &lt;Picker
                  selectedValue={this.state.initialOption}
                  onValueChange={this.handleChangeOption}
                  prompt='Options'}
                &gt;
                &lt;Picker.Item label='Please select an option...' value='0' /&gt;
                &lt;Picker.Item label='option 1' value='1' /&gt;
                &lt;Picker.Item label='option 2' value='2' /&gt;
              &lt;/Picker&gt;
  &lt;/View&gt;
  </code></pre>