update whatsnew

Author: Nico Cernek

doc/source/whatsnew/v1.0.0.rst

@@ -83,6 +83,7 @@ Backwards incompatible API changes
 
 - :class:`pandas.core.groupby.GroupBy.transform` now raises on invalid operation names (:issue:`27489`).
 - :class:`pandas.core.arrays.IntervalArray` adopts a new ``__repr__`` in accordance with other array classes (:issue:`25022`)
+- :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*