Remove QUALITY_IMPROVEMENTS.md, add NuGet badge, and add comprehensive SECURITY.md

[Copilot]

Plan for Repository Documentation Updates

• Remove QUALITY_IMPROVEMENTS.md file (as changes are captured in autogenerated release notes)
• Add NuGet shield badge to README.md for the DemaConsulting.SpdxTool package
• Create SECURITY.md file with:
  • Statement that only the latest version is supported
  • Instructions to use GitHub vulnerability report feature
• Verify all changes are correct
• Run build and tests to ensure no issues (all 97 tests pass, 0 warnings)
• Address code review feedback (fixed table formatting in SECURITY.md)
• Address PR comments:
  • Moved NuGet badge to last position and made it clickable link to nuget.org
  • Expanded SECURITY.md with additional standard sections (What to Expect, Security Update Process, Security Best Practices, Input Validation, Security Tools Used, Responsible Disclosure, Security Hall of Fame, Contact, Additional Resources)
• Fix cspell issue by adding 'sandboxed' to .cspell.json dictionary

Original prompt

Remove the QUALITY_IMPROVEMENTS.md file, as changes such as this are captured in the autogenerated release notes.

Please add a nuget shield for the DemaConsulting.SpdxTool package.

Please add a standard SECURITY.md file indicating that only the latest version is supported and that the Github vulnerability report feature should be used.

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[Malcolmnixon]

Please consider expanding the SECURITY.md file with additional standard sections:

• What to Expect
• Security Update Process
• Security Best Practices
• Input Validation
• Security Tools Used
• Responsible Disclosure
• Security Hall of Fame
• Contact
• Additional Resources

[Malcolmnixon]

Please put the NuGet Version as the last shield, and also make it a link which takes the user to the nuget.org site for the package.

[Copilot]

Done in c3292b4. The NuGet badge is now the last shield and links to https://www.nuget.org/packages/DemaConsulting.SpdxTool

[Copilot]

Please consider expanding the SECURITY.md file with additional standard sections:

• What to Expect
• Security Update Process
  ...

Done in c3292b4. Added all suggested sections to SECURITY.md:

• What to Expect
• Security Update Process
• Security Best Practices
• Input Validation
• Security Tools Used
• Responsible Disclosure
• Security Hall of Fame
• Contact
• Additional Resources