Create SECURITY.md for security policy

[kimmymuezza-droid]

Added a security policy document outlining supported versions and vulnerability reporting.

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Note

Introduces a security policy document for clarity on maintenance and reporting.

• Adds SECURITY.md with a supported versions table (5.1.x, 4.0.x supported; 5.0.x, < 4.0 unsupported)
• Includes a "Reporting a Vulnerability" section with guidance placeholders

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