Add README for NetBird Unraid plugin

Add detailed instructions for installing and using NetBird as a plugin for Unraid, including configuration steps and local building instructions.

README.md

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+# NetBird for Unraid
+
+Run [NetBird](https://netbird.io) — the open-source WireGuard mesh VPN — as a
+native Unraid OS plugin. The Unraid host itself becomes a peer on your
+NetBird network. No Docker container, no extra hop.
+
+> ⚠️ Preview release. Validated on Unraid 7.0+. Not yet listed in Community
+> Applications - see [Listing in CA](#listing-in-ca) below for the path to
+> get there.
+
+## Install
+
+In **Plugins → Install Plugin**, paste:
+
+```
+https://raw.githubusercontent.com/netbirdio/netbird-unraid/main/plugin/netbird.plg
+```
+
+Then open **Settings → Netbird** to configure.
+
+## What you get
+
+- The official upstream `netbird` Linux binary at `/usr/local/sbin/netbird`
+- An rc.d service (`/etc/rc.d/rc.netbird`) that survives reboots
+- Dynamix WebGUI pages: **Settings**, **Status**, **Info** + a Dashboard tile
+- Persistent identity and config on the USB flash drive, so reboots don't
+  forget who this peer is
+
+## How it works
+
+NetBird's upstream packaging assumes systemd. Unraid runs Slackware with
+plain init, so this plugin:
+
+1. Downloads the upstream tarball from `github.com/netbirdio/netbird/releases`,
+   pinned by SHA256.
+2. Drops the binary at `/usr/local/sbin/netbird`.
+3. Runs `netbird service run` from `/etc/rc.d/rc.netbird` (foreground daemon,
+   logging to `/var/log/netbird.log`).
+4. Symlinks NetBird's two state paths onto the flash drive so the identity
+   survives Unraid's ephemeral rootfs:
+
+   | NetBird default      | Symlinked to                          |
+   | -------------------- | ------------------------------------- |
+   | `/etc/netbird`       | `/boot/config/plugins/netbird/etc`    |
+   | `/var/lib/netbird`   | `/boot/config/plugins/netbird/lib`    |
+
+5. Adds an array-start event hook so the daemon comes up whenever the array
+   starts.
+
+## Building locally
+
+```bash
+./scripts/build.sh
+```
+
+Produces:
+
+- `dist/unraid-netbird-utils-<version>-noarch-1.txz`
+- `dist/netbird.plg` with the version and package SHA256 substituted in
+
+Upload the `.txz` to a GitHub release tagged with the same version, then
+commit the updated `plugin/netbird.plg` to `main` so users get the new
+version through normal plugin updates.
+
+## Credit
+
+The structure here is inspired by the official Unraid
+[Tailscale plugin](https://github.com/unraid/unraid-tailscale) by Derek
+Kaser, same general daemon+CLI+Dynamix shape. Plugin docs from the
+[mstrhakr/plugin-docs](https://github.com/mstrhakr/plugin-docs) project.