Added reference to issue #2353 in CONTRIBUTING (#2521)

* Python books: Formatting for multiple editions

* added ref to issue 2353

* .

CONTRIBUTING.md

@@ -38,7 +38,7 @@ By contributing you agree to respect the [Code of Conduct](https://github.com/Eb
 - if a link exists in multiple format, add a separate link with a note about each format
 - if a resource exists at different places on the Internet
     + use the link with the most authoritative source (meaning author's website is better than editor's website is better than third party website)
-    + if they link to different editions and you judge these editions are different enough to be worth keeping them, add a separate link with a note about each edition
+    + if they link to different editions and you judge these editions are different enough to be worth keeping them, add a separate link with a note about each edition (see [Issue #2353](https://github.com/EbookFoundation/free-programming-books/issues/2353) to contribute to the discussion on formatting.)
 - prefer atomic commits (one commit by addition/deletion/modification) over bigger commits. No need to squash your commits before submitting a PR. (We will never enforce this rule as it's just a matter of convenience for the maintainers)
 - if the book is older, include the publication date with the title. 
 - include the author name or names where appropriate. You can shorten author lists with "et al."