Add README.md files to directories for improved GitHub browsing experience

[Copilot]

This PR improves the documentation and user experience when browsing the GABP repository on GitHub by adding comprehensive README.md files to all major directories.

Problem

When users browse subdirectories like EXAMPLES/1.0/ or SCHEMA/1.0/methods/ on GitHub, they see only file listings without context about what the directory contains or how to use the files. This makes it difficult for developers to understand the repository structure and find relevant documentation.

Solution

Added 13 new README.md files that GitHub will automatically display when browsing directories:

• EXAMPLES/1.0/ and subdirectories - Explain example message types, naming conventions, and validation
• SPEC/1.0/ - Provide reading order and implementation guidance for specification documents
• SCHEMA/1.0/ and subdirectories - Document schema usage, validation tools, and organization
• CONFORMANCE/1.0/ and subdirectories - Explain conformance testing approach and expected behavior

Key Features

Each README.md provides:

• Clear explanations of the directory's purpose and contents
• Cross-references to related directories and files
• Practical examples of how to use the files (validation commands, code snippets)
• Consistent formatting following the repository's markdown standards

For example, browsing to EXAMPLES/1.0/handshake/ now shows a detailed explanation of the GABP handshake process, what each example file demonstrates, and which schemas to validate against.

Benefits

• Better discoverability - Users can understand content without reading source files
• Improved onboarding - New contributors get context about repository structure
• Enhanced navigation - Clear guidance on where to find specific information
• Consistent documentation - Unified approach across all major directories

The changes are purely additive and don't modify any existing functionality or break any existing workflows.

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