Write comprehensive README

[mrsumanbiswas]

Create a detailed README.md that includes:

• Project name, logo, and one‑line description
• Tech stack overview (Go, Flutter, PostgreSQL, …)
• Local development setup (clone, Docker, environment variables, run server & app)
• Link to full API documentation (docs/api-spec.md)
• Contribution guide (branch naming, commit style, PR process)
• Badges (build status, license, version)

This is the first thing new contributors and testers will see. Make it clear and welcoming.