add a GitHub PR template

.github/pull_request_template.md

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+## Checklist for your PR
+
+- [ ] please start with a draft PR and mark it as *ready for review* when you are confident that your implementation works as expected.
+- [ ] add a changelog entry in [CHANGELOG.md](https://github.com/at-gmbh/personio-py/blob/master/CHANGELOG.md) with a link to this PR.
+- [ ] make sure that all tests pass and add new test functions if your code is not covered by existing tests. Please use mock tests when network requests are required.
+- [ ] let someone review & approve your code, make changes where necessary.
+
+## Hints
+
+* name your branches accordingly: use the prefixes `bugfix/`, `feature/`, or `release/` and then a short title for your PR in kebab-case (lowercase words separated with dashes), e.g. `feature/my-awesome-feature`.
+* Please make sure that [pre-commit hooks](https://github.com/at-gmbh/personio-py#contributing) are enabled on your system to avoid common mistakes before you make a commit.
+* feel free to delete this template and replace it with your own description of the PR.