Updated whatsnew to reflect changes

Author: benthayer

doc/source/whatsnew/v0.20.0.txt

@@ -26,6 +26,8 @@ Check the :ref:`API Changes <whatsnew_0200.api_breaking>` and :ref:`deprecations
 New features
 ~~~~~~~~~~~~
 
+- Added subtraction from FrozenLists (:issue:`15475`)
+
 - Integration with the ``feather-format``, including a new top-level ``pd.read_feather()`` and ``DataFrame.to_feather()`` method, see :ref:`here <io.feather>`.
 - ``.str.replace`` now accepts a callable, as replacement, which is passed to ``re.sub`` (:issue:`15055`)