GitHub: Provide more context in pull request issue comment

Be explicit that the user should update their branch and re-push to
update the pull request.

Reviewed by:	imp
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51332

.github/workflows/checklist.yml

@@ -89,6 +89,7 @@ jobs:
               /* Loop for each key in "checklist". */
               for (const c in checklist)
                 msg += "- " + c + "<sup>" + checklist[c].join(", ") + "</sup>\n";
+              msg += "\nPlease review CONTRIBUTING.md, then update and push your branch again.\n"
 
               comment_func({
                 owner: context.repo.owner,