docs: Add Hashnode article link to English README

[shinpr]

Summary

• Add link to comprehensive Hashnode article about Sub agents and Context Engineering
• Article demonstrates real-world implementation with 770K tokens processed without context exhaustion
• Follows same structure as Japanese README.ja.md with Practical Guide section

Article Details

• Title: "Zero Context Exhaustion: Building Production-Ready AI Coding Teams with Claude Code Sub-agents"
• URL: https://shinpr.hashnode.dev/zero-context-exhaustion-building-production-ready-ai-coding-teams-with-claude-code-sub-agents
• Content: Explains boilerplate implementation, Sub agents architecture, and practical examples

Changes

• Added new "💡 Practical Guide" section between Claude Code Workflow and Development Rules Overview sections
• Maintains consistency with Japanese version structure