added one more PR checklist item

Author: Stephan Dilly

.github/PULL_REQUEST_TEMPLATE.md

@@ -12,4 +12,5 @@ It changes the following:
 I followed the checklist:
 - [ ] I added unittests
 - [ ] I ran `make check` without errors
-- [ ] I tested the overall application
+- [ ] I tested the overall application
+- [ ] I added an appropriate item to the changelog