Add README documentation

[piraz]

The sqlok project is missing comprehensive documentation. Users cannot understand the project's purpose, features, or how to use it without a
README.

Goals

• Create comprehensive README.md with project overview
• Document core features and architecture
• Provide quick start examples for users
• Include development and testing instructions
• List dependencies and CI/CD setup

Acceptance Criteria

• README.md exists at project root
• Includes overview explaining sqlok's purpose (PostgreSQL schema manager + query builder)
• Documents key features with code examples
• Contains quick start section with SELECT, INSERT examples
• Describes internal architecture (builder.go, schema/, cli/)
• Includes development setup (make test, CI/CD info)
• Lists all external dependencies
• Provides roadmap for future features