Update template with security section (#148)

https://ucsc-cgl.atlassian.net/browse/SEAB-5077

Author: Charles Overbeck

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,6 +4,9 @@ A description of the PR, should include a decent explanation as to why this chan
 **Issue**
 A link to a github issue or SEAB- ticket (using that as a prefix)
 
+**Security**
+If there are any concerns that require extra attention from the security team, highlight them here.
+
 Please make sure that you've checked the following before submitting your pull request. Thanks!
 
 - [ ] Ensure that the PR targets the correct branch. Check the milestone or fix version of the ticket.