Replace contributions.md with contributing.md and make it more standard.

CONTRIBUTING.md

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+# Contributing to Pixi
+## Introduction
+Thank you for your interest in contributing to Pixi! We value the contributions from our community and look forward to collaborating with you.
+
+## Code of Conduct
+Before contributing, please read our [Code of Conduct](https://github.com/prefix-dev/pixi/blob/main/CODE_OF_CONDUCT.md). We expect all our contributors to follow it to ensure a welcoming and inclusive environment.
+
+## Getting Started
+**Familiarize yourself with Pixi:** Understand its functionality and codebase.
+**Set up your environment:** Ensure you have the necessary tools and dependencies.
+
+## How to Contribute
+- **Find an Issue:** Look for open issues or create a new issue to discuss your idea.
+- **Fork the Repository:** Make a copy of the repository to your account.
+- **Create a Branch:** Create a branch in your fork to work on your changes.
+- **Develop and Test:** Implement your changes and make sure they are thoroughly tested.
+- **Write Clear Commit Messages:** Make your changes easy to understand.
+- **Create a Pull Request:** Submit your changes for review.
+
+> [!TIP]
+> Discuss your change in the issue before developing the solution when the design is not predefined.
+> This will help speedup the actual pull request review.
+
+## Pull Request Process
+- Ensure your code adheres to the project's coding standards, we use automated tooling for that, try `pixi run lint`.
+- Document new code using the Rust docstrings like we do in most places.
+- Update the `README.md` or documentation (`docs/`) with details of changes to the interface, if applicable.
+- Your pull request will be reviewed by maintainers, who may suggest changes.
+
+## Reporting Bugs
+- Use the issue tracker to report bugs.
+- Clearly describe the issue, including steps to reproduce the bug.
+- Include any relevant logs or error messages.
+
+## Feature Requests
+- Use the issue tracker to suggest new features.
+- Explain how the feature would be beneficial to the project.
+
+## Questions and Discussions
+For general questions, consider using our chat platform: [Discord](https://discord.gg/kKV8ZxyzY4)
+
+## Acknowledgments
+Your contributions are highly appreciated and will be credited accordingly.
+
+## License
+By contributing to Pixi, you agree that your contributions will be licensed under its [LICENSE](https://github.com/prefix-dev/pixi/blob/main/LICENSE).
+
+# Tips while developing on `pixi`
+
+## Pixi is a pixi project so use a preinstalled `pixi` to run the predefined tasks
+```shell
+pixi run build
+pixi run lint
+pixi run test
+pixi run test-all
+pixi run install # only works on unix systems as on windows you can't overwrite the binary while it's running
+```
+
+## Get your code ready for a PR
+We use [`pre-commit`](https://pre-commit.com/) to run all the formatters and linters that we use.
+If you have `pre-commit` installed on your system you can run `pre-commit install` to run the tools before you commit or push.
+If you don't have it on your system either use `pixi global install pre-commit` or use the one in your environment.
+```shell
+pixi run lint
+```
+
+When you commit your code, please try to come up with a good commit message.
+The maintainers (try to) use [conventional-commits](https://www.conventionalcommits.org/en/v1.0.0/).
+```shell
+git add FILES_YOU_CHANGED
+# This is the conventional commit convention:
+git commit -m "<type>[optional scope]: <description>"
+# An example:
+git commit -m "feat: add xxx to the pixi.toml"
+```