title: How to use django-smart-select
body: <p>Let's say I have the following model:</p>
  
  <p><code>class Location(models.Model)
      continent = models.CharField(max_length=20)
      country = models.ForeignKey(Country)
  </code></p>
  
  <p>I need to create a dependent dropdown so that when I select a continent I get all countries belonging to that continent. How should I do it?</p>