docs: Create root README.md

[KemingHe]

Problem

Repository needs comprehensive user guide that enables developers to understand, install, and use git-crypt-retro in under 5 minutes.

Proposed Solution

Create root README.md (target: under 200 lines) covering:

• Overview: What is git-crypt-retro and why it exists (retroactive encryption gap)
• Prerequisites: Git >= 2.36.0, Python >= 3.6, git-crypt, POSIX environment, backup requirements
• Shared Responsibility Model: Explicit table comparing git/git-filter-repo/git-crypt/git-crypt-retro responsibilities
• Tool Comparison Table: Feature/behavior matrix showing what each tool does
• Quick Start: Clone repo, verify prerequisites, run with --repo flag
• Critical Limitations: Level-2 heading covering:
  • GitHub cache persistence (requires Support contact)
  • GPG signatures stripped
  • All branches/tags rewritten
  • Remote tracking removed
• Usage: Command syntax, preview output, confirmation flow
• Post-Operation: Force push instructions, team coordination, verification
• Troubleshooting: Common errors with resolutions
• References: Link to LICENSE, CONTRIBUTING, SECURITY, design docs, KemingHe/git-crypt-retro-e2e-test

Exception to typical README line limits justified by single-stop user guide requirement.

Alternatives Considered

Splitting into README + detailed docs, but users in panic mode need everything immediately accessible.