Handle django rest framework HiddenField (#79)

* Added HiddenFieldInspector (#78)
* Updated docs + added test for HiddenField

Added HiddenField to testproj (no change in yaml as expected)

Closes #78.

src/drf_yasg/inspectors/field.py

@@ -465,11 +465,21 @@ class DictFieldInspector(FieldInspector):
         return NotHandled
 
 
+class HiddenFieldInspector(FieldInspector):
+    """Hide ``HiddenField``."""
+
+    def field_to_swagger_object(self, field, swagger_object_type, use_references, **kwargs):
+        if isinstance(field, serializers.HiddenField):
+            return None
+
+        return NotHandled
+
+
 class StringDefaultFieldInspector(FieldInspector):
     """For otherwise unhandled fields, return them as plain :data:`.TYPE_STRING` objects."""
 
     def field_to_swagger_object(self, field, swagger_object_type, use_references, **kwargs):  # pragma: no cover
-        # TODO unhandled fields: TimeField HiddenField JSONField
+        # TODO unhandled fields: TimeField JSONField
         SwaggerType, ChildSwaggerType = self._get_partial_types(field, swagger_object_type, use_references, **kwargs)
         return SwaggerType(type=openapi.TYPE_STRING)