docs: add status badges and a Quick start section

[daichirata]

Summary

Visual polish for the README top half — no functional changes.

• Status badges under the title: test workflow, latest release, Go version, license. The repo's state is now visible at a glance.
• Quick start section directly after the architecture diagram. A single Docker one-liner that reuses the reader's existing Application Default Credentials lets them verify gcsproxy works in 30 seconds before scrolling through Features / Installation / Usage.

Preview

# gcsproxy

[Test ✓] [Release v0.4.4] [Go 1.25] [License MIT]

A lightweight reverse proxy for Google Cloud Storage.
... description ...
... ASCII diagram ...

## Quick start

docker run --rm -p 8080:80 \
    -e GOOGLE_APPLICATION_CREDENTIALS=/cred.json \
    -v ~/.config/gcloud/application_default_credentials.json:/cred.json \
    ghcr.io/daichirata/gcsproxy:latest -b 0.0.0.0:80

curl http://localhost:8080/<your-bucket>/<your-object>

Test plan

• No code changes; existing tests unaffected
• Badge URLs follow GitHub / shields.io conventions; will resolve once the PR is merged and rendered on master

🤖 Generated with Claude Code