docs: added reference to gis serializer

added reference to GeoFeatureModelSerializer of django-rest-framework-gis

docs/api-guide/serializers.md

@@ -590,6 +590,11 @@ The following third party packages are also available.
 
 The [django-rest-framework-mongoengine][mongoengine] package provides a `MongoEngineModelSerializer` serializer class that supports using MongoDB as the storage layer for Django REST framework.
 
+## GeoFeatureModelSerializer
+
+The [django-rest-framework-gis][django-rest-framework-gis] package provides a `GeoFeatureModelSerializer` serializer class that supports GeoJSON both for read and write operations.
+
 [cite]: https://groups.google.com/d/topic/django-users/sVFaOfQi4wY/discussion
 [relations]: relations.md
 [mongoengine]: https://github.com/umutbozkurt/django-rest-framework-mongoengine
+[django-rest-framework-gis]: https://github.com/djangonauts/django-rest-framework-gis