Update README with NuGuard

[KanishkThamman]

Adds NuGuard (https://github.com/NuGuardAI/nuguard), an open-source
(Apache-2.0) CLI that:

• Generates an AI-SBOM (AIBOM) from a local codebase or Git repo
• Performs structural/supply-chain risk analysis
• Runs automated red-teaming for prompt injection, tool/MCP misuse, and
  data exfiltration
• Validates AI behavioral policy compliance (static + runtime)
• Exports findings as SARIF/JSON/Markdown with remediation suggestions

Installable via pip install nuguard.