Update PULL_REQUEST_TEMPLATE.md

updating PR template to encouraging linking to issues

Author: cmwilson21

PULL_REQUEST_TEMPLATE.md

@@ -6,6 +6,11 @@ A clear and concise description of what the problem or opportunity is. Link to a
 
 Describe the changes this pull request introduces.
 
+<!-- If there's an existing issue for your change, please link to it below next to "Closes".
+If there's _not_ an existing issue, please open one first to make it more likely that this update will be accepted. -->
+
+Closes:
+
 ### Task list
 
 - [ ] For workflow changes, I have verified the Actions workflows function as expected.