Add Pull Request template

.github/PULL_REQUEST_TEMPLATE/pull_request_template.md

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+#### Description
+
+- Reason for the Pull Request (PR)
+- Summary of the proposed changes
+
+#### Relation to open issues
+
+- Closes #XXX
+- ...
+
+#### Preliminary checks
+
+- [ ] I have gone through the [Contribution guidelines](https://protopipe.readthedocs.io/en/latest/contribute/index.html)
+- [ ] I have added or improved unit testing related to the proposed changes
+- [ ] I have updated or improved the documentation related to the proposed changes
+- [ ] All tests and documentation build pass without warnings or errors on my local setup
+
+#### Additional information
+
+...