Add whatsnew note

Author: makbigc

doc/source/whatsnew/v0.25.0.rst

@@ -40,6 +40,7 @@ Other Enhancements
 - :meth:`DataFrame.query` and :meth:`DataFrame.eval` now supports quoting column names with backticks to refer to names with spaces (:issue:`6508`)
 - :func:`merge_asof` now gives a more clear error message when merge keys are categoricals that are not equal (:issue:`26136`)
 - :meth:`pandas.core.window.Rolling` supports exponential (or Poisson) window type (:issue:`21303`)
+- :meth:`ExtensionArray.argsort` places the nan at the end of the sorted array (:issue:`21801`)
 
 .. _whatsnew_0250.api_breaking: