add drf-url-filters app to django-rest-framework in filtering docs (#4528)
add drf-url-filters app to django-rest-framework in filtering docs (#4528)

- enabling validtions on incoming query params with the ease of adding new filters as easy as adding a new key in a dict.
edx/django-rest-framework,uploadcare/django-rest-framework,dmwyatt/django-rest-framework,dmwyatt/django-rest-framework,jpadilla/django-rest-framework,jpadilla/django-rest-framework,ossanna16/django-rest-framework,kgeorgy/django-rest-framework,uploadcare/django-rest-framework,davesque/django-rest-framework,kgeorgy/django-rest-framework,davesque/django-rest-framework,jpadilla/django-rest-framework,edx/django-rest-framework,tomchristie/django-rest-framework,linovia/django-rest-framework,linovia/django-rest-framework,linovia/django-rest-framework,tomchristie/django-rest-framework,atombrella/django-rest-framework,kgeorgy/django-rest-framework,atombrella/django-rest-framework,ossanna16/django-rest-framework,dmwyatt/django-rest-framework,davesque/django-rest-framework,atombrella/django-rest-framework,edx/django-rest-framework,uploadcare/django-rest-framework,tomchristie/django-rest-framework,ossanna16/django-rest-framework