add further instructions to PR template

Author: karenetheridge

.github/pull_request_template.md

@@ -1,12 +1,14 @@
 <!--
 A good description should explain the problem the pull request addresses
 and give context to the reviewers to aid them in their reviews.
+If this addresses an open issue, please reference it in this format: GH #12345
+so that GitHub adds a link in that issue to this new pull request.
 -->
 TODO: fill description here
 
 -------------------------------------------------------------------------------
 <!--
-Significant changes to Perl must be documented in perldelta.
+Significant or noteworthy changes to Perl must be documented in pod/perldelta.pod.
 
 Consider if the changes in this pull request are worthy of a perldelta
 entry, then pick the appropriate line below and remove the others.