DOC: update RELEASE.rst

Author: y-p

doc/source/v0.13.0.txt

@@ -49,6 +49,8 @@ Bug Fixes
   - In ``to_json``, raise if a passed ``orient`` would cause loss of data because
     of a duplicate index (:issue:`4359`)
 
+  - Suppressed DeprecationWarning associated with internal calls issued by repr() (:issue:`4391`)
+
 See the :ref:`full release notes
 <release>` or issue tracker
 on GitHub for a complete list.