update whatsnew

Author: Nico Cernek

doc/source/whatsnew/v1.0.0.rst

@@ -274,6 +274,8 @@ New repr for :class:`~pandas.arrays.IntervalArray`
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 - :class:`pandas.arrays.IntervalArray` adopts a new ``__repr__`` in accordance with other array classes (:issue:`25022`)
+- :class:`pandas.core.groupby.GroupBy.transform` now raises on invalid operation names (:issue:`27489`).
+- :meth:`DataFrame.merge` now preserves right frame's row order when executing a right merge. Older versions do not preserve row order on right merge (:issue:`27453`)
 
 *pandas 0.25.x*