chore: add PR template (#2143)

Author: SteveLauC

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,7 @@
+## What does this PR do
+
+## Checklist:
+
+- [ ] I have read `CONTRIBUTING.md`
+- [ ] I have written necessary tests and rustdoc comments
+- [ ] A change log has been added if this PR modifies nix's API