Add whatsnew note

Author: Albert Villanova del Moral

doc/source/whatsnew/v0.20.0.txt

@@ -505,11 +505,7 @@ Bug Fixes
 - Bug in ``DataFrame.to_stata()`` and ``StataWriter`` which produces incorrectly formatted files to be produced for some locales (:issue:`13856`)
 
 
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 - Bug in ``DataFrame.boxplot`` where ``fontsize`` was not applied to the tick labels on both axes (:issue:`15108`)
 - Bug in ``Series.replace`` and ``DataFrame.replace`` which failed on empty replacement dicts (:issue:`15289`)
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+- Bug in ``DataFrame.fillna()`` where the argument ``downcast`` was ignored when fillna value was of type dict instead of a scalar (:issue:`15277`)