Added release notes

Author: radarhere

docs/releasenotes/9.1.0.rst

@@ -48,6 +48,13 @@ breaking backwards compatibility.
 Other Changes
 =============
 
+ImageShow temporary files
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When calling :py:meth:`~PIL.Image.Image.show` or using :py:mod:`~PIL.ImageShow`,
+a temporary file is created from the image. Pillow will no longer delete these files,
+and instead leave it to the operating system to do so.
+
 Image._repr_pretty_
 ^^^^^^^^^^^^^^^^^^^