Doc: Add shelve custom serialization to What's New in 3.15

Author: furkanonder

Doc/whatsnew/3.15.rst

@@ -661,6 +661,9 @@ shelve
 * Added new :meth:`!reorganize` method to :mod:`shelve` used to recover unused free
   space previously occupied by deleted entries.
   (Contributed by Andrea Oliveri in :gh:`134004`.)
+* Add support for custom serialization and deserialization functions
+  in the :mod:`shelve` module.
+  (Contributed by Furkan Onder in :gh:`99631`.)
 
 
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