Add a URL to support a TODO comment.

Summary: Provide a link to a "resolved" [[http://bugs.xdebug.org/view.php?id=1041 | Xdebug issue]] to support a TODO comment. Further information can be found at http://derickrethans.nl/dead-code.html.

Test Plan: Read the article.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9696