title: custom filtering in django rest framework
body: <p>I am trying to implement custom filtering in my code and went through the documentation. But I couldnt understand the following snippets from the docs.</p>
  <pre><code>class UserFilter(django_filters.FilterSet):
      class Meta:
          model = User
          fields = ['username', 'last_login']
  
  # or
  
  class UserFilter(django_filters.FilterSet):
      class Meta:
          model = User
          fields = {
              'username': ['exact', 'contains'],
              'last_login': ['exact', 'year__gt'],
          }
  </code></pre>
  <p>The doc says we can use either one the first or second one but what is the exact difference?? What is the meaning of contains and year_gt?</p>