title: Show attribute of foreign key in add_form in Django
body: <p>Is it possible show an attribute of foreign key in add_form django?</p>
  
  <p>For example:</p>
  
  <pre><code>#models.py
  class Test(models.Model):
      name = models.CharField(max_length=60, db_column='Name')  # Field name made lowercase.
      product=models.ForeignKey(Product, on_delete=models.DO_NOTHING)
  
  class Product(models.Model):
      id = models.AutoField(db_column='Id', primary_key=True)
      description = models.CharField(max_length=60, db_column='Description')  
  
  #admin.py
  class TestAdmin(admin.ModelAdmin):
      fields = ['name','product','get_description']
      readonly_fields = ['get_description']
  
      def get_description(self):
          return self.product.description
  </code></pre>
  
  <p>Naturally this code raise exception the ErrorFields in 'get_description'.</p>
  
  <p>But, does exist a way to show the 'get_description' when I insert an entry of model Test?</p>
  
  <p>Thanks</p>