Added whatsnew for issue 15193

Author: Nicholas Ver Halen

doc/source/whatsnew/v0.20.0.txt

@@ -632,3 +632,4 @@ Bug Fixes
 - Bug in ``pd.melt()`` where passing a tuple value for ``value_vars`` caused a ``TypeError`` (:issue:`15348`)
 - Bug in ``.eval()`` which caused multiline evals to fail with local variables not on the first line (:issue:`15342`)
 - Bug in ``pd.read_msgpack`` which did not allow to load dataframe with an index of type ``CategoricalIndex`` (:issue:`15487`)
+- Bug in ``DataFrame.pivot_table`` where ``dropna=True`` would not drop columns from a categorical series to columns (:issue:`15193`)