title: Django-filter filter by related fields
body: <p>I have two tables.</p>
  
  <pre><code>class Writer(models.Model)
      name = model.CharField()
      ...
  
  class Article(models.Model)
      name = model.CharField()
      writer = model.ForeignKey('Writer', related_name="relationship") 
      ...
  </code></pre>
  
  <p>I wanted to build some API endpoints getting the writer list, but this should be filterable by Article id.
  I am using django-filter. So:</p>
  
  <pre><code>class WriterViewSet(viewsets.ReadOnlyModelViewSet):
     filter_backend = [filters.djangoFilterBackend],
     filter_class = WriterFilter
  
  
  class WriteFilter(django_filters.rest_framework.FilterSet):
  ....
  </code></pre>
  
  <p>So my concern is, how can I define WriteFilter to filter Writer by the article?</p>