add a whatsnew entry in v1.4.0

Author: liwh

doc/source/whatsnew/v1.4.0.rst

@@ -129,6 +129,7 @@ Other enhancements
 - :meth:`DataFrame.__pos__`, :meth:`DataFrame.__neg__` now retain ``ExtensionDtype`` dtypes (:issue:`43883`)
 - The error raised when an optional dependency can't be imported now includes the original exception, for easier investigation (:issue:`43882`)
 - Added :meth:`.ExponentialMovingWindow.sum` (:issue:`13297`)
+- :meth:`DataFrame.corr` now accept the argument ``calculate_diagonal`` to allow results returned from callable to be used as diagonal elements of the correlation matrix instead of setting them to ones (:issue:`25781`)
 
 .. ---------------------------------------------------------------------------