Enhance README with ScanCode Action details

[divyanshrai1]

Description

This PR addresses issue #12 by adding a comprehensive "Understanding ScanCode Action" section to the README.

Changes Made

Added New Section: "Understanding ScanCode Action"

This section includes:

1. What Does This Action Do? - Explains that the action scans for packages, copyrights, licenses, and optionally vulnerabilities
2. What Does "Build Passed" Mean? - Clarifies that a green checkmark means the scan completed successfully, NOT that the code is compliant
3. Where Are My Scan Results? - Step-by-step instructions to find and download artifacts
4. How to Interpret Results - Explains the different output formats (JSON, XLSX, SPDX, CycloneDX)
5. Enforcing Compliance - Shows how to use check-compliance to fail builds on policy violations

Updated Table of Contents

• Added links to all new subsections for easy navigation

Motivation

As mentioned in #12, users were confused about what "build passed" means and how to interpret scan results. This documentation makes it clear that:

• A passed build only means the scan completed without errors
• Users must manually review scan results to understand what was found
• The action provides data, not automatic compliance decisions (unless policies are configured)

Testing

• Verified all markdown links work correctly
• Checked rendering in GitHub preview
• Confirmed table of contents auto-links function properly

Looking forward to your feedback! This is my first contribution to open source, so please let me know if any changes are needed.