Document GitHub Copilot configuration for repository maintainers

[Copilot]

The repository already has comprehensive Copilot instructions (.github/copilot-instructions.md and .github/instructions/*.md) but lacks documentation explaining the configuration to maintainers and contributors.

Changes

• Added .github/README.md - Documents the Copilot configuration structure, explains when to update repository-wide vs path-specific instructions, and provides instruction file format requirements

• Updated contributing.md - Added "GitHub Copilot Configuration" section informing contributors that AI assistance is available and linking to maintainer documentation

Current Configuration

.github/
├── README.md                                    # 🆕 Maintainer documentation
├── copilot-instructions.md                      # Repository-wide (421 lines)
└── instructions/
    ├── custom_auth_product.instructions.md      # Product requirements
    ├── custom_auth_structure.instructions.md    # Code structure
    └── custom_auth_tech.instructions.md         # Technical standards

The existing instructions follow GitHub best practices with proper YAML frontmatter (applyTo: "**/custom_auth/**") and cover build procedures, code standards, and PR guidelines. This PR makes the configuration discoverable and maintainable without modifying the instruction content.

Original prompt

Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.