add whatsnew entry for simple_efficiency division-by-zero fix

Author: aman-coder03

docs/sphinx/source/whatsnew/v0.13.3.rst

@@ -14,6 +14,9 @@ Deprecations
 
 Bug fixes
 ~~~~~~~~~
+* Fix a division-by-zero condition in
+  :py:func:`pvlib.transformer.simple_efficiency`
+  when ``load_loss = 0``. (:pull:`2646`)
 
 
 Enhancements