docs: add SECURITY.md (#27)

Author: Gtax2006

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 0.1.x   | :white_check_mark: |
+
+## Reporting a Vulnerability
+
+We take security seriously. If you discover a vulnerability in this gas oracle toolkit:
+
+1. **Do not open a public issue** — use GitHub Security Advisories instead
+2. Email security@kcolbchain.com with details
+3. Allow 48 hours for acknowledgment, 14 days for fix
+
+## Scope
+
+This project provides gas price prediction and oracle services.
+Vulnerabilities in price prediction algorithms, RPC handling, or data sources
+will be triaged and fixed.
+
+## Disclosure Policy
+
+We follow coordinated disclosure. Patches will be credited in release notes.