Add SECURITY.md (private vuln reporting policy)

SECURITY.md

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+# Security policy
+
+Thanks for taking the time to look at this. Even small tools can introduce real
+risk — this one binds a privileged UDP port and reconfigures a network adapter
+on the host — so vulnerability reports are very welcome.
+
+## Supported versions
+
+Only the latest tagged release on GitHub is supported. Older versions will not
+get fixes; please upgrade first.
+
+## How to report a vulnerability
+
+**Please do not open a public issue** for security-sensitive findings. Use one
+of these private channels instead:
+
+1. **Preferred:** GitHub's private security advisories.
+   Go to the [Security tab](../../security/advisories/new) of this repo and
+   click "Report a vulnerability". GitHub will route it to me privately.
+2. **Email:** `engelgardt2024@gmail.com` with the subject prefix `[security]`.
+
+Please include:
+- The version of `dhcpsrv` you tested (output of the startup banner is enough).
+- Steps to reproduce, ideally with a packet capture or a short script.
+- An assessment of impact (LAN-only? remote? admin needed? etc.).
+
+## What to expect
+
+- Acknowledgement within **3 business days**.
+- A first technical reply within **7 business days**.
+- A fix and a public advisory once the issue is resolved. Reporters are
+  credited unless they prefer to stay anonymous.
+
+## Out of scope
+
+- DoS by flooding the DHCP server on the local link (it's a small tool meant
+  for direct-cable / single-switch use; flooding your own laptop is your call).
+- Behavior when run **without** administrator privileges (the tool refuses to
+  start in that case anyway).
+- Issues that require the attacker to already control the user's machine.