Update pull request template to require 'upstream' PR links for PRs not against 'main'

Author: bstansberry

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,4 +2,6 @@ Thanks for submitting your Pull Request!
 
 Please delete this text, and add a link to the Jira issue solved by this PR.
 
-Remember to use the Jira issue ID in the PR title, and any commits.
+If this PR is not for the 'main' branch you must add a link to the equivalent change in 'main'.
+
+Remember to use the Jira issue ID in the PR title and any commits.