Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contribute to Cleo
+
+## Project Management
+
+The Cleo project is managed by [Poetry](https://python-poetry.org/). You need to install it first:
+
+```bash
+pipx install poetry
+```
+
+For other installation methods, please refer to [Poetry's documentation](https://python-poetry.org/docs/#installation).
+
+After the installation, install the project and dependencies:
+
+```bash
+poetry install
+```
+
+## Run the Tests
+
+```bash
+poetry run pytest
+```
+
+## Code Style and Linters
+
+We use [Black](https://github.com/psf/black) to format the code and [Ruff](https://github.com/charliermarsh/ruff) as the linter.
+They are all integrated into [pre-commit](https://pre-commit.com/). You can enable it by:
+
+> NOTICE: `pre-commit` is declared as one of development dependencies of Cleo. If you don't have `pre-commit` installed globally, prepend the commands in this section with `poetry run`
+
+```bash
+pre-commit install
+```
+
+and run the checks by:
+
+```bash
+pre-commit run --all-files
+```
+
+### News fragments
+
+When you make changes such as fixing a bug or adding a feature, you must add a news fragment describing
+your change. News fragments are placed in the `news/` directory, and should be named according to this pattern: `<issue_num>.<issue_type>.md` (e.g., `566.bugfix.md`).
+
+> NOTICE: If your change doesn't have an issue, please use PR number in place of `<issue_num>`
+
+#### Issue Types
+
+- `break`: Breaking changes
+- `feat`: Features & Improvements
+- `bugfix`: Bug fixes
+- `docs`: Changes to documentation
+- `deps`: Changes to dependencies
+- `removal`: Removals or deprecations in the API
+- `misc`: Miscellaneous changes that don't fit any of the other categories
+
+The contents of the file should be a single sentence in past tense that describes your changes (e.g., `Added CONTRIBUTING.md file.`).
+See entries in the [Change Log](/CHANGELOG.md) for more examples.
+
+

news/331.docs.md

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+Added CONTRIBUTING.md file.