add missing plugin entrypoint, add plugins.ini mypy config with plugins specified

mypy_django_plugin/plugins/postgres_fields.py

@@ -9,3 +9,7 @@ class PostgresFieldsPlugin(Plugin):
     def get_base_class_hook(self, fullname: str
                             ):
         return determine_type_of_array_field
+
+
+def plugin(version):
+    return PostgresFieldsPlugin