Create comprehensive README.md for project onboarding

[Copilot]

All reviewers (DevOps/Frontend/PM) flagged missing README as blocking effective team onboarding. Created comprehensive documentation from minimal placeholder.

Changes

• Project overview: Learning platform concept, "snack-sized" developer content with Chart.js visualizations
• Tech stack: HTML5/CSS3/JavaScript (ES6+), Chart.js
• Setup instructions: Browser direct-open method + npm dev server workflow
• Directory structure: Complete tree with planned architecture (docs/, src/, .github/)
• Contribution guidelines: Links to CONTRIBUTING.md and CODE_OF_CONDUCT.md (marked as planned)

Structure

developer-snacks/
├── README.md              # This file
├── src/                   # Source code
│   ├── styles/
│   └── scripts/
├── docs/                  # Architecture, onboarding guides
└── .github/               # CI/CD, templates

README follows standard OSS format with badges, emoji section markers, and bilingual content (Japanese primary, English tech terms).

Original prompt

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This section details on the original issue you should resolve

<issue_title>[最優先] README.md の作成</issue_title>
<issue_description>## 概要
プロジェクトの入口となる README.md を作成する。

背景

• 全てのレビュアー（DevOps/Frontend/PM）が最重要として指摘
• 新メンバーのオンボーディング時間短縮に不可欠

含めるべき内容

• プロジェクト概要・目的
• 技術スタック（HTML/CSS/JS, Chart.js）
• ローカルでの実行方法
• ディレクトリ構成
• コントリビューション方法へのリンク

優先度

🔴 最優先（全員一致）</issue_description>

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

• Fixes [最優先] README.md の作成 #1

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