ENH:3715 - add documentation

Author: nchmura4

doc/source/whatsnew/v0.20.0.txt

@@ -49,8 +49,8 @@ Other enhancements
 ^^^^^^^^^^^^^^^^^^
 
 - ``pd.read_excel`` now preserves sheet order when using ``sheetname=None`` (:issue:`9930`)
-
 - ``pd.cut`` and ``pd.qcut`` now support datetime64 and timedelta64 dtypes (issue:`14714`)
+- ``DataFrame.asfreq()`` now accepts a ``fill_value`` option to fill missing values during resampling (:issue:`3715`).
 
 .. _whatsnew_0200.api_breaking: