docs: expand the README (features, configuration, deployment, contributing)

[peb-peb]

Summary

The current README.md is a minimal quick-start. Since the repo is public and community-facing, it should be expanded into a proper project README that helps newcomers understand, run, configure, and contribute to the project.

Why

• First thing visitors see on GitHub.
• New contributors need context (what it does, how it's structured, how to configure S3).
• Currently there's no mention of the watermarking behaviour, environment variables, tech stack, or how to contribute.

Suggested sections

• What it is — one-liner + a short paragraph (drag & drop image → watermarked, shareable URL).
• Demo / screenshot — a screenshot or GIF of the uploader in action.
• Features — watermarking, HEIC support, WebP output, S3-backed sharing.
• Tech stack — Next.js 15, React 19, Tailwind v4, sharp, AWS S3.
• Getting started — prerequisites (Node, pnpm), install, .env setup.
• Configuration — document every var in .env.example (AWS S3 credentials, bucket, etc.).
• Project structure — brief map of src/ (app, components, lib/server).
• Deployment — build/start, plus note about outputFileTracingIncludes for the watermark asset.
• Contributing — link to the issue/PR templates once they exist.
• License — link to the LICENSE file once added.

Acceptance criteria

• README renders cleanly on GitHub and covers setup, configuration, and contribution at a glance.

Related

• Depends loosely on #LICENSE and the issue/PR template issues (link them once created).