Rewrite README: user-facing documentation

[YANGCHUNHONG3000]

The current README is entirely a contributing guide — it opens with how to fork, clone, and set up a dev environment. But there's no project introduction, no installation guide, no entity list, no usage instructions. Users who land here have no idea what Indeklima does or how to get started.

This PR rewrites the README to be user-facing:

• What it does — plain language explanation of indoor climate monitoring, severity scores, mold risk, ventilation recommendations
• Installation — HACS and manual
• Configuration — config flow walkthrough
• Entities — per-room and hub entities listed
• Dashboard — sidebar panel and Lovelace cards
• Troubleshooting — points to the existing TROUBLESHOOTING.md
• Contributing — points to the existing CONTRIBUTING.md

The existing CONTRIBUTING.md already covers the dev setup (which was duplicated in the old README), so this removes the duplication and keeps the README focused on users.

[YANGCHUNHONG3000]

Hey! Just wanted to add some context — I opened issue #6 to explain the thinking behind this PR. No pressure at all — if you like the direction I'm happy to iterate, if not, totally fine to close. Either way, cool project! 🏠