Update README.md to enhance project descriptions and usage examples

[hsaito]

This pull request updates the README.md to provide clearer documentation and usage instructions for the My Number library. The changes improve project descriptions, add feature and usage examples, and include build and test instructions.

Documentation improvements:

• Expanded the project overview to clarify that the repository provides tools for both validating and generating Japanese My Numbers in .NET.
• Added feature lists and code examples for MyNumberNET, including validation and check digit calculation methods.
• Detailed CLI usage, commands, and arguments for MyNumberNET_CLI.
• Provided instructions for running unit tests in MyNumberNET_Test.
• Included build instructions and a license section for easier setup and compliance.