chore: Add a pull request template

.github/CONTRIBUTING.md

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+See the contributing guide in the [docs](https://citric.readthedocs.io/en/latest/contributing/code-of-conduct.html) for more information.

.github/PULL_REQUEST_TEMPLATE/pull_request_template.md

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+## Summary
+
+<!-- What's the purpose of the change? What does it do, and why? -->
+
+## Test Plan
+
+<!-- How was it tested? -->
+
+## Checklist
+
+<!-- Put an `x` in the boxes that apply. You can also fill these out after creating the PR. -->
+
+- [ ] My pull request has a descriptive title.
+- [ ] I have read the [CONTRIBUTING] guide.
+- [ ] I have added tests that prove my fix is effective or that my feature works.
+- [ ] If appropriate, I have added necessary documentation.
+- [ ] For user-facing changes, refactorings, performance improvements or documentation updates, I have added a changelog entry using the [Changie] tool.
+
+For both the title of the PR and the changelog entry, prefer simple past tense or constructions with "now". For example:
+
+  - Added `Client.invite_participants()`
+  - `Client.user_activation_settings()` now accepts a `user_activation_settings` keyword argument
+
+[CONTRIBUTING]: https://citric.readthedocs.io/en/latest/contributing/code-of-conduct.html
+[Changie]: https://changie.dev/