title: Display possible values (choices) of SlugRelatedField in drf-yasg OpenAPI and Swagger views
body: <p>I have several models that I use as enums (basically, the model is just a name and a slug), like currencies and countries etc, and I'm trying to show the available choices in drf-yasg without success.
  My last attempt was adding this to the serializer's Meta class:</p>
  
  <pre><code>swagger_schema_fields = {
      'currency': {'enum': list(Currency.objects.values_list('slug', flat=True))}
  }
  </code></pre>
  
  <p>But of course it failed miserably - not only it didn't show the enum values, it also broke the serializer (because it used the strings instead of the actual model).</p>
  
  <p>Is there any way of doing this?</p>