DOC: Mention rpy2 breakage in release notes

Author: y-p

doc/source/release.rst

@@ -416,6 +416,9 @@ API Changes
     copy thru chained assignment is detected, settable via option ``mode.chained_assignment``
   - test the list of ``NA`` values in the csv parser. add ``N/A``, ``#NA`` as independent default
     na values (:issue:`5521`)
+  - The refactoring involving``Series`` deriving from ``NDFrame`` breaks ``rpy2<=2.3.8``. an Issue
+    has been opened against rpy2 and a workaround is detailed in :issue:`5698`. Thanks @JanSchulz.
+
 
 Internal Refactoring
 ~~~~~~~~~~~~~~~~~~~~

doc/source/v0.13.0.txt

@@ -879,6 +879,9 @@ to unify methods and behaviors. Series formerly subclassed directly from
 
    - ``Series(0.5)`` would previously return the scalar ``0.5``, instead this will return a 1-element ``Series``
 
+   - This change breaks ``rpy2<=2.3.8``. an Issue has been opened against rpy2 and a workaround
+     is detailed in :issue:`5698`. Thanks @JanSchulz.
+
 - Pickle compatibility is preserved for pickles created prior to 0.13. These must be unpickled with ``pd.read_pickle``, see :ref:`Pickling<io.pickle>`.
 
 - Refactor of series.py/frame.py/panel.py to move common code to generic.py