Add OpenSSF Best Practices badge

[pmady]

Apply for the OpenSSF Best Practices badge and add it to the README.

What to do:

1. Go to https://www.bestpractices.dev/en and create a new project entry
2. Fill out the questionnaire for gpu-mcp-server (most criteria are already met)
3. Add the badge to the top of README.md
4. Document any remaining criteria gaps in a follow-up issue

Why this matters:

• Required for AAIF project submission (Growth stage)
• Shows the project follows security best practices
• Builds trust with potential adopters

Files to edit: README.md