Add whatsnew note for simple_efficiency division-by-zero fix

Author: aman-coder03

docs/sphinx/source/whatsnew/v0.15.1.rst

@@ -14,7 +14,9 @@ Deprecations
 
 Bug fixes
 ~~~~~~~~~
-
+* Fix a division-by-zero condition in
+  :py:func:`~pvlib.transformer.simple_efficiency` when ``load_loss = 0``.
+  (:issue:`2645`)
 
 Enhancements
 ~~~~~~~~~~~~
@@ -42,4 +44,5 @@ Maintenance
 
 Contributors
 ~~~~~~~~~~~~
+* Aman Srivastava (:ghuser:`aman-coder03`)