Add offline/air-gapped download instructions to README

Clarifies that the installer script can be downloaded on another machine and transferred via USB, while noting the kiosk machine still needs internet access during install for apt/npm packages.

Readme.md

@@ -59,6 +59,38 @@ The installer will guide you through configuration during setup.
 
 ---
 
+## Offline / Air-Gapped Download
+
+If the kiosk machine can't reach GitHub directly (no browser, restrictive proxy, or you just prefer to grab the script on another computer and carry it over via USB), download it ahead of time instead of using the `curl`/`wget` one-liner above.
+
+> **Note:** This only avoids needing internet access *to fetch the script*. The installer itself still requires the kiosk machine to have internet access while it runs — it uses `apt` to install packages, pulls Node.js from NodeSource, and runs `npm install` to fetch Electron (~120MB). There is currently no fully air-gapped/offline package bundle.
+
+**On a machine with internet access:**
+
+```bash
+# Option A: download just the latest installer script
+LATEST=$(curl -fsSL https://api.github.com/repos/outis1one/ubuntu-based-kiosk/contents \
+  | grep -oP 'ubuntu-based-kiosk-v[0-9.]+\.sh' \
+  | grep -v beta | sort -V | tail -1)
+wget "https://github.com/outis1one/ubuntu-based-kiosk/raw/main/$LATEST"
+
+# Option B: download the whole repo as a ZIP (includes all installer versions and addon scripts)
+wget https://github.com/outis1one/ubuntu-based-kiosk/archive/refs/heads/main.zip
+unzip main.zip
+```
+
+Copy the downloaded `.sh` file (or the extracted ZIP contents) to a USB drive, then on the kiosk machine:
+
+```bash
+# Mount the USB drive and copy the script over, then:
+chmod +x ubuntu-based-kiosk-v*.sh
+./ubuntu-based-kiosk-v*.sh
+```
+
+The kiosk machine still needs a working internet connection (ethernet, or WiFi configured during Ubuntu install) for the script to complete.
+
+---
+
 ## Core Features
 
 ### Multi-Site Management