[JOSS] Create CONTRIBUTING.md

[KedoKudo]

Summary

Create contribution guidelines for community contributors.

Requirements

• Create CONTRIBUTING.md in repository root
• Document development setup
• Explain PR process
• Reference code style and testing requirements

Suggested Content

Sections

1. Getting Started

   • Prerequisites (Rust, pixi, etc.)
   • Development setup
   • Running tests

2. Making Changes

   • Fork and branch workflow
   • Code style (rustfmt, clippy)
   • Test requirements
   • Documentation expectations

3. Pull Request Process

   • PR template
   • Review process
   • CI requirements

4. Reporting Issues

   • Bug report template
   • Feature request template

5. Code of Conduct

   • Link to CODE_OF_CONDUCT.md or inline

References

• Good CONTRIBUTING examples from similar projects
• JOSS community guidelines requirement