Add changelog entry

Author: larrybradley

CHANGES.rst

@@ -22,6 +22,14 @@ New Features
   more accurate transformations for directed regions such as ellipses and
   rectangles. [#650, #651]
 
+- Added a ``use_ellipse`` keyword to the ``to_pixel`` and ``to_sky``
+  methods of circular and circular annulus region classes that allows
+  the user to specify whether to return an elliptical region instead of
+  a circular region when converting between pixel and sky coordinates.
+  This is useful for cases where the WCS is distorted or has different
+  pixel scales along different axes and the circular region would be
+  significantly distorted in pixel or sky coordinates. [#652]
+
 Bug Fixes
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