add note in whatsnew 1.2

Author: arw2019

doc/source/whatsnew/v1.2.0.rst

@@ -222,6 +222,7 @@ Performance improvements
 - Performance improvement in :meth:`GroupBy.agg` with the ``numba`` engine (:issue:`35759`)
 - Performance improvements when creating :meth:`pd.Series.map` from a huge dictionary (:issue:`34717`)
 - Performance improvement in :meth:`GroupBy.transform` with the ``numba`` engine (:issue:`36240`)
+- Performance improvement in :meth:`pd.to_datetime` with non-`ns` time unit for `float` `dtype` columns (:issue:`20445`)
 
 .. ---------------------------------------------------------------------------