Add project documentation files

[RAprogramm]

Add standard project documentation files for professional open-source library.

Objectives

• Create CHANGELOG.md for version history
• Create CONTRIBUTING.md for contributors
• Create SECURITY.md for security policy
• Improve project maintainability

CHANGELOG.md

• Auto-generate from git tags and commits
• Follow Keep a Changelog format
• Include release dates and version numbers
• Categorize changes (Added, Changed, Fixed, etc.)

CONTRIBUTING.md

• Development setup instructions
• Code style guidelines (rustfmt, clippy)
• Testing requirements
• PR submission process
• AI Development Protocol v2.1 reference

SECURITY.md

• Vulnerability reporting process
• Supported versions policy
• Security contact information
• Response timeline expectations

Implementation

• Create all three files in project root
• Add REUSE compliant headers
• Link from README.md
• Update REUSE.toml annotations

Benefits

• Professional project appearance
• Clear contribution guidelines
• Transparent security policy
• Better community engagement
• Required for crates.io best practices