ENH:3715 - add documentation

Author: nchmura4

doc/source/whatsnew/v0.20.0.txt

@@ -31,7 +31,7 @@ Other enhancements
 ^^^^^^^^^^^^^^^^^^
 
 - ``pd.read_excel`` now preserves sheet order when using ``sheetname=None`` (:issue:`9930`)
-
+- ``DataFrame.asfreq()`` now accepts a fill_value option to fill missing values during resampling (:issue:`3715`).
 
 .. _whatsnew_0200.api_breaking: