Create SECURITY.md (#1892)

Added a security policy document outlining reporting procedures for vulnerabilities.

Author: matthewdouglas

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+We provide security updates for the latest stable minor release line.
+
+| Version  | Supported |
+| -------- | --------- |
+| 0.49.x   | ✅        |
+| < 0.49.x | ❌        |
+
+> Note: Pre-releases, development builds, and commits on `main` are not considered supported release versions. If you believe you have found a vulnerability in unreleased code, please still report it following the process below.
+
+## Reporting a Vulnerability
+
+Please report security issues **privately** using the GitHub Security Advisory tool to create a new draft advisory:
+
+- https://github.com/bitsandbytes-foundation/bitsandbytes/security/advisories/new
+
+Do not open a public GitHub issue for security-sensitive reports.
+
+### What to include
+
+To help us triage and respond quickly, please include:
+
+- A clear description of the issue and potential impact
+- Affected version(s) and environment details (OS, GPU type, CUDA version, Python version, PyTorch version, etc)
+- Steps to reproduce (ideally a minimal proof of concept)
+- Any relevant logs, crash traces, or screenshots
+- Any known mitigations or workarounds
+
+## Response process
+
+We will review reports filed via GitHub Security Advisories and collaborate with the reporter in the advisory thread to:
+
+- Confirm and reproduce the report
+- Assess severity and affected versions
+- Identify mitigations and/or prepare a fix
+- Coordinate any follow-up needed prior to broader communication