Added the note with example

Author: KiranAkadas

doc/source/development/contributing.rst

@@ -950,9 +950,13 @@ the expected correct result::
         assert_frame_equal(pivoted, expected)
 
 .. note::
-
-   Add the GitHub Issue Number as a Comment to the New Tests.
-
+ 
+   Add the GitHub issue number as a comment to the new tests.
+   For Example - For a test solving issue number 20172, write  
+     | # GH 20172 
+     | def test_apply_categorical_with_nan_values(self, series):
+     |       s = pd.Series(series, dtype="category")
+     |       ...
 
 Transitioning to ``pytest``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~