Add multiple choice filtering example to docs

Author: kimbriancanavan

docs/filtering.rst

@@ -115,6 +115,11 @@ create your own ``Filterset`` as follows:
     class AnimalFilter(django_filters.FilterSet):
         # Do case-insensitive lookups on 'name'
         name = django_filters.CharFilter(lookup_expr=['iexact'])
+        # Allow multiple genera to be selected at once
+        genera = django_filters.MultipleChoiceFilter(field_name='genus',
+                                                     choices=(('Canis', 'Canis'),
+                                                              ('Panthera', 'Panthera'),
+                                                              ('Seahorse', 'Seahorse')))
 
         class Meta:
             model = Animal
@@ -127,6 +132,23 @@ create your own ``Filterset`` as follows:
         all_animals = DjangoFilterConnectionField(AnimalNode,
                                                   filterset_class=AnimalFilter)
 
+If you were interested in selecting all dogs and cats, you might query as follows:
+
+.. code::
+
+    query {
+      allAnimals(genera: ["Canis", "Panthera"]) {
+        edges {
+          node {
+            id,
+            name
+          }
+        }
+      }
+    }
+
+
+
 The context argument is passed on as the `request argument <http://django-filter.readthedocs.io/en/master/guide/usage.html#request-based-filtering>`__
 in a ``django_filters.FilterSet`` instance. You can use this to customize your
 filters to be context-dependent. We could modify the ``AnimalFilter`` above to