Add pull request template (#119)

Author: facutuesca

.github/pull_request_template.md

@@ -0,0 +1,14 @@
+## Summary
+
+<!-- What's the purpose of the change? What does it do, and why? -->
+
+## Checklist
+<!-- Remove the ones that don't apply to this PR -->
+
+### Making a release
+- [ ] Bump the version in `src/pypi_attestations/__init__.py`
+- [ ] Add a new subheading in the CHANGELOG for the new version
+- [ ] Add a link at the bottom of the CHANGELOG for the diff of the new version
+
+### Adding new public-facing APIs
+- [ ] Expose the new types/functions in `src/pypi_attestations/__init__.py`