Added changelog entry.

Author: semohr

docs/changelog.rst

@@ -70,6 +70,8 @@ Other changes:
   Autogenerated API references are now located in the `docs/api` subdirectory.
 * :doc:`/plugins/substitute`: Fix rST formatting for example cases so that each
   case is shown on separate lines.
+* Refactored library.py file by splitting it into multiple modules within the 
+  beets/library directory.
 
 2.3.1 (May 14, 2025)
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