add docstring

Author: dcherian

cf_xarray/groupers.py

@@ -7,6 +7,10 @@
 
 @dataclass
 class FlagGrouper(UniqueGrouper):
+    """
+    Grouper object that allows convenient categorical grouping by a CF flag variable.
+    """
+
     def factorize(self, group) -> EncodedGroups:
         if "flag_values" not in group.attrs or "flag_meanings" not in group.attrs:
             raise ValueError(