Update CONTRIBUTING.rst (vacanza#2085)

CONTRIBUTING.rst

@@ -11,9 +11,10 @@ Basics
 
 When contributing with fixes and new features, please start forking/branching
 from the `dev branch`_ to work on the latest code and reduce merging issues.
-If you add/change holiday official dates or names you must include references to
+If you add/change holiday official dates or names your code must include references to
 all sources (government sites, archived web pages, wiki pages, etc) you've used
-while working on this PR. Contributed PRs_ are required to include valid test
+while working on this PR. That could be done either as a ``References`` section update or 
+as a comment on the relevant part of the code. Contributed PRs_ are required to include valid test
 coverage in order to be merged. Please don't hesitate to ask for help if you
 need one with the tests.