Create PULL_REQUEST_TEMPLATE.md

Author: julienschmidt

PULL_REQUEST_TEMPLATE.md

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+### Description
+Please explain the changes you made here.
+
+### Checklist
+- [ ] Code compiles correctly
+- [ ] Created tests, if possible
+- [ ] All tests passing
+- [ ] Extended the README / documentation, if necessary
+- [ ] Added myself / the copyright holder to the AUTHORS file