Add comprehensive README.md with known issues and contribution guidelines

[Copilot]

Replaced generic Flutter template README with production-ready documentation. Explicitly documents that SMS transaction parsing is non-functional and invites contributions.

Changes

• Project overview: Description of Finenzo as smart expense tracker with feature list (multi-wallet, analytics, biometric security, CSV/PDF/Excel export)
• Setup instructions: Prerequisites, dependencies, build_runner workflow for Drift database generation
• Technology stack: Flutter 3.8.1, Provider, Drift (SQLite), telephony, fl_chart, local_auth
• Known Issues section: SMS transaction parsing marked as broken - inconsistent format parsing, limited bank pattern support, Android version reliability issues
• Contributing section: Fork/PR workflow, priority areas (SMS parsing, internationalization, bank support, iOS, testing), development guidelines

Key Section

## ⚠️ Known Issues

### Message Transaction Feature
The SMS transaction parsing feature is currently **not working as expected**. While the app can read SMS messages and attempt to parse transaction details from mobile money services, there are known issues with:
- Inconsistent SMS format parsing
- Limited pattern matching for various banks and services
- Reliability issues on different Android versions

We are aware of this limitation and are working to improve it. **Contributions are welcome!**

Original prompt

Add readme.md for this project. mention the fact that message transaction feature doesn't work and contributions are welcome

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