Add "note on ai contributions"

Author: Jibola

.github/pull_request_template.md

@@ -5,6 +5,7 @@
 If you are an external contributor and there is no JIRA ticket associated with your change, then use your best judgement
 for the PR title. A MongoDB employee will create a JIRA ticket and edit the name and links as appropriate.
 
+Note on AI Contributions:
 We do not accept pull requests that are primarily or substantially generated by AI tools (ChatGPT, Copilot, etc.).
 All contributions must be written and understood by human contributors.
 -->