Updated the whatsnew for issue 15021

Author: Nicholas Ver Halen

doc/source/whatsnew/v0.20.0.txt

@@ -149,6 +149,7 @@ Other enhancements
 - ``Series/DataFrame.asfreq()`` have gained a ``fill_value`` parameter, to fill missing values (:issue:`3715`).
 - ``Series/DataFrame.resample.asfreq`` have gained a ``fill_value`` parameter, to fill missing values during resampling (:issue:`3715`).
 - ``pandas.tools.hashing`` has gained a ``hash_tuples`` routine, and ``hash_pandas_object`` has gained the ability to hash a ``MultiIndex`` (:issue:`15224`)
+- ``groupby(..).resample()`` is now able to use the ``on`` argument. (:issue:`15021`)
 
 .. _ISO 8601 duration: https://en.wikipedia.org/wiki/ISO_8601#Durations