Add CONTRIBUTING.md

[ukorvl]

We need to provide some standard for developers how to make feature requests, report bugs and contribute to the reposiotry.
We already have https://github.com/NilFoundation/nil.js/blob/6454208fe8665ec40c342dca6eb1baa3ef10ede7/.github/CONTRIBUTING.md but there is only a description of how to work with changesets. What should be added as well:

• Description of how to setup repository to start development
• Naming conventions
• How to setup tests and how to test your code with real cluster interaction
• Maybe we should move CONTRIBUTING.md to the root of the repository, it looks like a common practice.

[starc007]

Hey @ukorvl
I've created PR #117 that addresses all the requested improvements to our contribution guidelines:

• ✅ Moved CONTRIBUTING.md to the root directory for better visibility
• ✅ Added comprehensive setup instructions for local development
• ✅ Documented naming conventions based on our current codebase patterns
• ✅ Added detailed testing guidelines, including local cluster setup
• ✅ Included clear workflow for issues and PRs
• ✅ Updated README.md with a new contributing section

Please review the changes and let me know if anything needs adjustment.