Extend PR template a bit

Author: patrickbkr

.github/pull_request_template.md

@@ -3,4 +3,6 @@ Checklist
 (Feel free to delete this text once you went through it.)
 
 - Did you add an entry to the `Changes` file?
+- If there is no `{{NEXT}}` above the latest released version in `Changes`, add
+  it.