title: django ModelMultipleChoiceField set initial values
body: <p>I have the following code:</p>
  
  <pre><code>category = forms.ModelMultipleChoiceField(
      label="Category",
      queryset=Category.objects.order_by('name'),
      widget=forms.Select(
          attrs={
              'placeholder': 'Product Category', 'class': 'form-control'}),
      required=True
  )
  </code></pre>
  
  <p>how do I set an initial value in the select box like "Choose a category" so that the select box should have a list of categories with the initial value being "Choose a category"</p>