DOC: added release note

Author: TomAugspurger

doc/source/whatsnew/v0.21.0.txt

@@ -334,6 +334,7 @@ I/O
 - Bug in :func:`read_csv` in which memory management issues in exception handling, under certain conditions, would cause the interpreter to segfault (:issue:`14696`, :issue:`16798`).
 - Bug in :func:`read_csv` when called with ``low_memory=False`` in which a CSV with at least one column > 2GB in size would incorrectly raise a ``MemoryError`` (:issue:`16798`).
 - Bug in :func:`read_csv` when called with a single-element list ``header`` would return a ``DataFrame`` of all NaN values (:issue:`7757`)
+- Bug in :meth:`DataFrame.to_csv` defaulting to 'ascii' encoding in Python 3, instead of 'utf-8' (:issue:`17097`)
 - Bug in :func:`read_stata` where value labels could not be read when using an iterator (:issue:`16923`)
 - Bug in :func:`read_html` where import check fails when run in multiple threads (:issue:`16928`)