Add what's new entry for update following pydata#8542

Author: spencerkclark

doc/whats-new.rst

@@ -48,6 +48,10 @@ Bug fixes
   encoding of time-like variables (:issue:`7132`, :issue:`8230`, :issue:`8432`,
   :pull:`8575`). By `Spencer Clark <https://github.com/spencerkclark>`_ and
   `Mattia Almansi <https://github.com/malmans2>`_.
+- Raise an informative error if dtype encoding of time-like variables would
+  lead to integer overflow or unsafe conversion from floating point to integer
+  values (:issue:`8542`, :pull:`8575`).  By `Spencer Clark
+  <https://github.com/spencerkclark>`_.
 
 
 Documentation