docs: add CONTRIBUTING guide and expand README

[leodiegues]

This pull request introduces significant improvements to project documentation, focusing on onboarding, usage, and contribution guidelines. The most notable changes are the addition of a comprehensive CONTRIBUTING.md file and a major rewrite of the README.md to provide clearer instructions, usage examples, and detailed reference material for users and contributors.

Documentation improvements:

• Added a new CONTRIBUTING.md file with detailed instructions on development setup, code style, testing, and pull request workflow to help new contributors get started and follow project standards.
• Completely rewrote the README.md to include:
  • A warning about early development status and API instability.
  • Expanded installation instructions (shell script, Cargo, building from source).
  • A quick start guide with step-by-step examples.
  • Detailed documentation for all CLI commands, their options, and usage examples.
  • Manifest file format reference, including required fields and file mapping conventions.
  • Explanation of supported remote git sources and file structure conventions.
  • Reference to the new CONTRIBUTING.md for further contribution details.