title: Nested routes using drf-extensions are not documented in API docs
body: <p>I'm writing API in DRF and had need to add nested routes. 
  Since <code>drf-extensions</code> are specified in official documentation I tend to use this package and it seemed to be working, but the documentation generated by Django Rest Framework doesn't include nested Viewset(route).</p>
  
  <p>The documentation is generated using <code>include_docs_urls</code> from <code>rest_framework.documentation</code></p>
  
  <p>I would really like to have my nested endpoint described in generated documentation.
  Is it possible?</p>
  
  <p>djangorestframework==3.9.0</p>
  
  <p>drf-extensions==0.5.0</p>
  
  <p>Python2.7</p>