Add WhatsNew entry

Author: kernc

doc/source/whatsnew/v0.24.0.txt

@@ -80,6 +80,7 @@ Other Enhancements
   <https://pandas-gbq.readthedocs.io/en/latest/changelog.html#changelog-0-5-0>`__.
   (:issue:`21627`)
 - New method :meth:`HDFStore.walk` will recursively walk the group hierarchy of an HDF5 file (:issue:`10932`)
+- :class:`SparseDataFrame` and :class:`SparseSeries` support value assignment (:issue:`21818`)
 - :func:`read_html` copies cell data across ``colspan``s and ``rowspan``s, and it treats all-``th`` table rows as headers if ``header`` kwarg is not given and there is no ``thead`` (:issue:`17054`)
 - :meth:`Series.nlargest`, :meth:`Series.nsmallest`, :meth:`DataFrame.nlargest`, and :meth:`DataFrame.nsmallest` now accept the value ``"all"`` for the ``keep`` argument. This keeps all ties for the nth largest/smallest value (:issue:`16818`)
 - :class:`IntervalIndex` has gained the :meth:`~IntervalIndex.set_closed` method to change the existing ``closed`` value (:issue:`21670`)