chore: Update README.md

[vblagoje]

Why:

The modifications aim to improve user guidance and usability for the OpenAPI-LLM library. By including badges and a structured table of contents in the README, users quickly grasp the project's status and navigate its features. Also, the additional emphasis on OpenAPI specification validation educates users on best practices, promoting effective use and integration with various Large Language Models (LLMs).

What:

• Added badges for PyPI version, downloads, Python version compatibility, test status, coverage, and license, increasing library visibility and credibility.
• Updated the README to include a comprehensive table of contents, enhancing document navigation.
• Clarified features, including enhanced authentication mechanisms and support for both YAML and JSON OpenAPI specifications.
• Expanded the installation section with supported Python versions.
• Introduced a section on OpenAPI specification validation with recommended tools and example code.

How can it be used:

• Quickly navigate the updated README to access information about the library's features, installation, and usage.
• Utilize the new validation guidance:

  from openapi_spec_validator import validate_spec
  import yaml
  
  with open('your_spec.yaml', 'r') as f:
      spec_dict = yaml.safe_load(f)
  validate_spec(spec_dict)
• Verify compatibility with Python versions (>= 3.8) before installation.

How did you test it:

Testing isn't explicitly covered in the README changes, but automated GitHub workflows (indicated by badges) likely ensure integration and functionality. The README itself, being a documentation change, inherently doesn't require static testing but clarity and accessibility were the primary validation focus.

Notes for the reviewer:

Ensure the badge URLs and testing workflows are correctly linked and reflect accurate information. The validation code snippets must be verified for completeness and correctness, avoiding potential user confusion. Also, check that the README maintains consistency and that all newly introduced sections link correctly in the table of contents.