title: Custom Model Field required=False on ModelForm
body: <p>I have the following custom model field:</p>
  
  <pre><code>script = S3EnabledFileField(bucket=settings.MEDIA_BUCKET, upload_to=...)
  </code></pre>
  
  <p>In my forms.py, I am using a ModelForm:</p>
  
  <pre><code>class CreateProductionForm(ModelForm):
      name = forms.CharField(required=True)
  
  class Meta:
      model = Production
      fields = ('name', 'script')
  </code></pre>
  
  <p>This makes <code>script</code> a required form field. How would I make script an optional field (I don't know how to define a form field for the customized model field).</p>