[PATCH] README: Add Quick Start section for beginner onboarding

[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
Location: Chengalpattu, Tamil Nadu, India
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