docs: update README for v1.2.0 — 5-framework coverage, 754 skills

v1.1.0 — 753 cybersecurity skills

What's in this release:
- 753 structured cybersecurity skills across web security, pentesting, DFIR,
  threat intelligence, cloud security, OT/SCADA, AI security, and more
- 5 skills upgraded with full step-by-step content (amcache forensics,
  Linux audit log analysis, OAuth token theft detection, DevSecOps scanning,
  privileged session monitoring)
- index.json bumped to version 1.1.0
- MITRE ATT&CK Enterprise coverage: 14/14 tactics, 291+ techniques
- Compatible with Claude Code, GitHub Copilot, Cursor, Windsurf, Gemini CLI

Install:
  npx skills add mukul975/Anthropic-Cybersecurity-Skills

Apache 2.0 licensed.

docs: add star history graph to README