add issue number

Author: charlesdong1991

pandas/tests/test_strings.py

@@ -3427,6 +3427,7 @@ def test_method_on_bytes(self):
 
     @pytest.mark.skipif(compat.PY2, reason='not in python2')
     def test_casefold(self):
+        #GH25405
         casefolded = Series(['ss', NA, 'case', 'ssd'])
         s = Series(['ß', NA, 'case', 'ßd'])
         result = s.str.casefold()