title: Changing the default placeholder in a Django model form for a ForeignKey
body: <p>I have my code set up as follows:</p>
  
  <p><strong>models.py</strong></p>
  
  <pre><code>class Category(models.Model):
      name = models.CharField(max_length=255)
  
  class Product(models.Model):
      category = models.ForeignKey(Category)
  </code></pre>
  
  <p><strong>forms.py</strong></p>
  
  <pre><code>class ProductForm(forms.ModelForm):
      class Meta:
          model = Product
          fields = ('category',)
          widgets = {
              #'category' : forms.TextInput(attrs={'placeholder' : 'Select the category'}),
          }
  </code></pre>
  
  <p>Basically, right now, without the widgets part, I get <code>--------</code> as the placeholder for this field. However, I want to get 'Select the category', how can I do this? Thanks!</p>