Update README.md

[bbyalcinkaya]

This PR adds a polished README to introduce Komet and help developers get started quickly.

What's included:

• Overview: Clear explanation of Komet's fuzzing and formal verification capabilities for Soroban smart contracts, and a GIF to showcase Komet in action.
• Installation guide: Step-by-step setup using kup (our Nix-based package manager)
• Quick start: Property test code example with structure explanation
• Workflow guidance: When to use fuzzing vs. formal verification during development
• Features & benefits: Highlights of Komet's testing capabilities and security guarantees
• Documentation links: Pointers to guides, examples, and video demo
• Roadmap: Upcoming features including advanced Soroban Wasm debugger with IDE integrations
• Community resources: Discord, homepage, and support links

The README includes placeholder for a demo GIF (assets/komet-demo.gif) to showcase Komet in action.