Added SECURITY.md file

- Consistent with other public repos
- Points at our docs on how to report security issues

SECURITY.md

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+# Reporting Security Vulnerabilities
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+Potential security vulnerabilities can be reported directly us at `security@ghost.org`. The Ghost Security Team communicates privately and works in a secured, isolated repository for tracking, testing, and resolving security-related issues.
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+The full, up-to-date details of our security policy and procedure can always be found in our documentation:
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+https://docs.ghost.org/security/
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+Please refer to this before emailing us. Thanks for helping make Ghost safe for everyone 🙏.