Added note to 'contributing.rst file', telling users to append GH Issue… (pandas-dev#28907)

Author: S3rbane

doc/source/development/contributing.rst

@@ -949,6 +949,9 @@ the expected correct result::
 
         assert_frame_equal(pivoted, expected)
 
+Please remember to add the Github Issue Number as a comment to a new test.
+E.g. "# brief comment, see GH#28907"
+
 Transitioning to ``pytest``
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