Add checklist item for opening an issue before PR

Author: darsor

.github/pull_request_template.md

@@ -7,5 +7,6 @@ this change.
 # Checklist
 
 - [ ] I have reviewed this project's [contribution guidelines](https://github.com/SystemRDL/PeakRDL-regblock-vhdl/blob/main/CONTRIBUTING.md)
+- [ ] I have opened an issue to discuss the desired change.
 - [ ] This change has been tested and does not break any of the existing unit tests. (if unable to run the tests, let us know)
 - [ ] If this change adds new features, I have added new unit tests that cover them.