Enhance README with Examples, API Overview, and Contribution Details

[odaydebian]

Dear Maintainers,

I am a Debian contributor passionate about open-source projects and would love to contribute to Go-Nmap. This pull request enhances the README.md file with the following modifications:

• Added usage examples: A code snippet demonstrating how to parse Nmap XML data.
• Expanded API overview: Detailed descriptions of core structs and functions for easier integration.
• Improved structure: Organized sections for better readability.
• Added contribution guidelines: Clear steps to encourage community involvement.
• Updated resources: Included links to relevant tools and references.

These changes aim to make the project more accessible and engaging for developers. I hope they align with the project's goals.

Thank you for maintaining this valuable library! I look forward to your feedback.

Best regards,
Marcos Rodrigues de Carvalho (aka oday) <marcosrcarvalho42@gmail.com>