add _columns to Model.update()

docs/releases.md

@@ -9,6 +9,8 @@
   *  `exclude: Union[Set, Dict, None]` -> set/dict of relations to exclude from save, those relation won't be saved even with `follow=True` and `save_all=True`. 
      To exclude nested relations pass a nested dictionary like: `exclude={"child":{"sub_child": {"exclude_sub_child_realtion"}}}`. The allowed values follow
      the `fields/exclude_fields` (from `QuerySet`) methods schema so when in doubt you can refer to docs in queries -> selecting subset of fields -> fields.
+*  `Model.update()` method now accepts `_columns: List[str] = None` parameter, that accepts list of column names to update. If passed only those columns will be updated in database.
+   Note that `update()` does not refresh the instance of the Model, so if you change more columns than you pass in `_columns` list your Model instance will have different values than the database!
 *  `Model.dict()` method previously included only directly related models or nested models if they were not nullable and not virtual, 
    now all related models not previously visited without loops are included in `dict()`. This should be not breaking
    as just more data will be dumped to dict, but it should not be missing.