Add SECURITY.md

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Security fixes are applied to the latest release only. We do not backport
+fixes to older versions.
+
+| Version | Supported |
+|---------|-----------|
+| 1.15+ | ✅ |
+| <1.15 | ❌ |
+
+## Reporting a Vulnerability
+
+**Please do not report security vulnerabilities through public GitHub issues.**
+
+Use GitHub's private vulnerability reporting instead:
+1. Go to the **Security** tab of this repository
+2. Click **Report a vulnerability**
+3. Fill in the details and submit
+
+### What to include
+
+A useful report tells us:
+- Which component or file is affected
+- What an attacker can do (impact) and under what conditions
+- A minimal reproduction case or proof-of-concept if you have one
+- Whether you believe it is remotely exploitable
+
+You do not need a working exploit to report. An incomplete report is better
+than no report.
+
+## What to expect
+
+This is a volunteer-maintained open-source project. We will do our best to
+respond in a reasonable timeframe, but cannot commit to specific deadlines.
+
+We ask that you give us a fair opportunity to investigate and address the
+issue before any public disclosure. If you have not heard back after
+**90 days**, feel free to follow up or proceed with disclosure at your
+discretion.
+
+## Scope
+
+In scope:
+- Remote code execution, memory corruption, or denial-of-service via crafted
+  radio packets
+- Authentication or encryption bypasses
+- Vulnerabilities in the packet routing or path handling logic
+
+Out of scope:
+- Physical access attacks (e.g., JTAG, UART extraction of keys)
+- Regulatory compliance (duty cycle, frequency restrictions)
+- Jamming or other physical-layer radio interference
+- Issues in third-party libraries (RadioLib, Crypto, etc.) — report those
+  upstream
+- "Best practice" suggestions without a demonstrated attack path