Add Local Setup guide for contributors (#54)

[TheekshanaCN]

What this PR does

Adds a detailed Local Setup guide in the README for contributors to run Celtrix locally and test changes.

Closes #54

How to test

1. Fork and clone the repo.
2. Follow the instructions in the README section "Local Development Setup".
3. Ensure the CLI links globally and scaffolds a test app successfully.

Notes

• Includes instructions for Node/npm versions, dependency installation, and linking CLI.
• Optional notes for future testing and troubleshooting included.

[TheekshanaCN]

Hi @Guruprasadh-R 👋, just checking in on this PR. Would love your review whenever you get a chance. Thanks! 🙏