docs: add English README for international / expat audience

[redchupa]

Adds `README.en.md` alongside the Korean README for HACS Default discovery + international visitors arriving via search.

Why

When this lands on HACS Default + community posts, the audience is broader than Korean residents alone:

• Korean expats abroad monitoring family-home utilities or disaster alerts back home
• Foreigners living in Korea running HA, fluent enough in Korean to use the actual services but more comfortable reading English docs
• International HA users integrating Korean air-quality / weather data into a global setup (e.g., overseas Koreans tracking PM2.5 in Seoul)

A literal translation of the Korean README would miss several pain points specific to this audience:

1. Agency-name disambiguation — multiple Korean government portals are involved (NMC, KMA, KECO, NEIS, MOIS, Seoul Metropolitan Government). English readers need explicit context.
2. Untranslated Korean GUI labels — HA shows the Korean labels as-is (`한국 컴포넌트 키트`, `최신 안전알림`, `수용가번호`). The English version leaves these as-is and adds English context alongside, so the reader knows what to type/look for in the actual HA UI.
3. 시도 / 시군구 / 읍면동 explanation — administrative hierarchy needs a one-time gloss for non-Koreans.

What's in it

Same structure as the Korean README:

• 5-minute quickstart (API-key-free Safety Alert demo)
• 13 services overview table
• 🔑 API key issuance guide with direct portal-search deep links (clickable 👉 links pre-filled with Korean search keywords — browsers auto-encode)
• ⚙️ Registration / reconfiguration flow
• 🎁 Entities-you-get reference table
• 🤖 LLM voice-query examples
• ❓ FAQ (10 items)
• 🔄 Update intervals
• ⚠️ Known limitations (honest disclosure)
• 🐛 Troubleshooting
• Requirements, Disclaimer, License

Top-of-Korean-README language toggle

Korean README now shows at the top:

```
🇰🇷 한국어 (이 페이지) · 🇬🇧 English README
```

GitHub renders both .md files; readers click the flag to switch.

🤖 Generated with Claude Code