Added whatsnew entry

Author: PhyNerd

doc/source/whatsnew/v0.21.1.txt

@@ -87,6 +87,7 @@ I/O
 - :func:`read_parquet` now allows to specify the columns to read from a parquet file (:issue:`18154`)
 - :func:`read_parquet` now allows to specify kwargs which are passed to the respective engine (:issue:`18216`)
 - Bug in parsing integer datetime-like columns with specified format in ``read_sql`` (:issue:`17855`).
+- Bug in :meth:`DataFrame.to_msgpack` when serializing data of the numpy.bool_ datatype (:issue:`18390`)
 
 
 Plotting