adding whatsnew note

Author: realead

doc/source/whatsnew/v1.2.0.rst

@@ -482,6 +482,7 @@ Performance improvements
 - faster ``dir`` calls when many index labels, e.g. ``dir(ser)`` (:issue:`37450`)
 - Performance improvement in :class:`ExpandingGroupby` (:issue:`37064`)
 - Performance improvement in :meth:`pd.DataFrame.groupby` for ``float`` ``dtype`` (:issue:`28303`), changes of the underlying hash-function can lead to changes in float based indexes sort ordering for ties (e.g. :meth:`pd.Index.value_counts`)
+- Performance improvement in :meth:`pd.isin` for inputs with more than 1e6 elements
 
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