Add comprehensive README documentation

[Copilot]

The repository had only a minimal README stub with basic setup instructions. This adds complete project documentation.

Documentation Added

• Project overview: Architecture description, microservices design (C++ crawler/indexer, Python BM25 ranker, Rails interface)
• Technology stack table: Component rationale (PostgreSQL, Redis, RocksDB, Docker)
• Installation guide: Prerequisites, environment setup, Docker Compose workflow
• API documentation: Ranker service endpoints (/health, /search) with request/response formats
• Development workflows: Individual service execution, log viewing, database access
• Troubleshooting: Common Docker/connectivity issues and resolutions
• Architecture diagrams: Online (search) and offline (crawling/indexing) data flows

Example API Usage

# Search query
curl "http://localhost:5000/search?q=your+query"

# Response
{
  "query": "your query",
  "results": [
    {"id": 1, "url": "...", "title": "...", "score": 4.52}
  ],
  "meta": {"count": 10, "latency_ms": 23.45}
}

Expands README from 25 to 465 lines with production-ready documentation.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Make a readme file for this oss</issue_title>
<issue_description>complete the readme of this project properly . </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Make a readme file for this oss #15

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