Document new included_fields argument to diff_against

Author: RealOrangeOne

docs/history_diffing.rst

@@ -20,3 +20,5 @@ This may be useful when you want to construct timelines and need to get only the
     delta = new_record.diff_against(old_record)
     for change in delta.changes:
         print("{} changed from {} to {}".format(change.field, change.old, change.new))
+
+``diff_against`` also accepts 2 arguments ``excluded_fields`` and ``included_fields`` to either explicitly include or exclude fields from being diffed.