Overhaul README with badges, quick start, and skill categories

CONTRIBUTING.md

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+# Contributing to Anthropic-Cybersecurity-Skills
+
+## How to add a new skill
+
+1. Create a new directory: `skills/your-skill-name/`
+2. Add a `SKILL.md` file with required YAML frontmatter:
+   ```yaml
+   ---
+   name: your-skill-name
+   description: >-
+     Clear description of what this skill does and when
+     an AI agent should activate it. Include keywords.
+   domain: cybersecurity
+   subdomain: [category]
+   tags: [tag1, tag2, tag3]
+   version: "1.0"
+   author: your-github-username
+   license: Apache-2.0
+   ---
+   ```
+3. Write clear, step-by-step instructions in the Markdown body using these sections:
+   - ## When to Use
+   - ## Prerequisites
+   - ## Workflow (numbered steps with real commands)
+   - ## Key Concepts (table)
+   - ## Tools & Systems
+   - ## Common Scenarios
+   - ## Output Format
+4. (Optional) Add supporting files:
+   - `references/standards.md` — Real standard numbers, CVE refs, NIST/MITRE links
+   - `references/workflows.md` — Deep technical procedure
+   - `scripts/process.py` — Real working helper script
+   - `assets/template.md` — Real filled-in checklist/template
+5. Submit a PR with title: `Add skill: your-skill-name`
+
+## Skill quality checklist
+- [ ] Name is lowercase with hyphens (kebab-case), 1–64 characters
+- [ ] Description is clear and includes agent-discovery keywords
+- [ ] Instructions are actionable with real commands and tool names
+- [ ] Domain and subdomain are set correctly
+- [ ] Tags include relevant tools, frameworks, and techniques
+
+## Subdomains
+Choose the most appropriate subdomain for your skill:
+- web-application-security
+- network-security
+- penetration-testing
+- red-teaming
+- digital-forensics
+- malware-analysis
+- threat-intelligence
+- cloud-security
+- container-security
+- identity-access-management
+- cryptography
+- vulnerability-management
+- compliance-governance
+- zero-trust-architecture
+- ot-ics-security
+- devsecops
+- soc-operations
+- incident-response
+- phishing-defense
+- ransomware-defense
+- api-security
+- mobile-security
+- endpoint-security
+- threat-hunting
+
+## Code of Conduct
+This project follows the [Contributor Covenant](CODE_OF_CONDUCT.md). By participating, you agree to uphold this code.
+
+## License
+By contributing, you agree that your contributions will be licensed under Apache-2.0.