Update whats-new.rst

Author: ColemanTom

doc/whats-new.rst

@@ -64,6 +64,8 @@ Bug fixes
   By `Alex Goodman <https://github.com/agoodm>`_ and `Deepak Cherian <https://github.com/dcherian>`_.
 - Fix :py:meth:`DataArray.plot.pcolormesh` which now works if one of the coordinates has str dtype  (:issue:`6775`, :pull:`7612`).
   By `Michael Niklas <https://github.com/headtr1ck>`_.
+- Improve error message when trying to open a file which you do not have permission to read (:issue:`6523`, :pull:`7629`).
+  By `Thomas Coleman <https://github.com/ColemanTom>`_.
 
 Documentation
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