[PATCH] README: Add Quick Start section with beginner-friendly build, test, verify, and mount guide

[Aayushmaan Chakraborty]

Hi EROFS maintainers,

As a prospective GSoC 2026 contributor interested in low-level
filesystem work, here's a small documentation improvement to
README.md:

- Added a prominent Quick Start section early for better onboarding
- Full beginner workflow: build → mkfs.erofs → fsck → erofsfuse mount → cleanup
- Compressed example + multithreading note
- Shields badges and upstream/community links

Patch attached (generated from my fork:
https://github.com/Aayushmaan-24/erofs-utils/tree/feature/readme-quick-start).

All commands verified locally. Happy to revise or split changes.

Thanks for considering!
Aayushmaan
GitHub: Aayushmaan-24

signed off by: Aayushmaan Chakraborty <aayushmaan.chakraborty at gmail.com>