adding whatsnew

Author: realead

doc/source/whatsnew/v0.24.0.txt

@@ -235,7 +235,7 @@ Other API Changes
   a ``KeyError`` (:issue:`21678`).
 - Invalid construction of ``IntervalDtype`` will now always raise a ``TypeError`` rather than a ``ValueError`` if the subdtype is invalid (:issue:`21185`)
 - Trying to reindex a ``DataFrame`` with a non unique ``MultiIndex`` now raises a ``ValueError`` instead of an ``Exception`` (:issue:`21770`)
--
+- :class:`Float64HashTable` handles zeros/signed zeros and all flavors of NaNs consistently: it is no longer possible to have both, zero and signed-zero, as keys at the same time in a table, also there can be at most one NaN-key in a table (:issue:`21866`)
 
 .. _whatsnew_0240.deprecations: