Add link to Classy DRF in docs (#5683)
Add link to Classy DRF in docs (#5683)

Closes #5636 as per https://github.com/encode/django-rest-framework/issues/5636#issuecomment-348143281
kgeorgy/django-rest-framework,kgeorgy/django-rest-framework,tomchristie/django-rest-framework,kgeorgy/django-rest-framework,tomchristie/django-rest-framework,tomchristie/django-rest-framework