Add SECURITY policy

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+If you discover a security vulnerability, please report it privately via GitHub's security advisory feature:
+
+1. Go to the **Security** tab
+2. Click **Report a vulnerability**
+3. Provide details about the vulnerability
+
+Please **do not** open a public issue for security vulnerabilities.
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 2.0.x   | :white_check_mark: |
+| < 2.0   | :x:                |
+
+## Security Best Practices
+
+- Always download binaries from official [Releases](https://github.com/y0sy4/tg-ws-proxy-go/releases)
+- Verify checksums when available
+- Keep your version up to date