docs/usage: New file

Document offline USB upgrades.

Signed-off-by: Colin Walters <[email protected]>

docs/usage.md

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+---
+nav_order: 3
+---
+
+# Managing bootc systems
+
+## Performing offline updates via USB
+
+In a usage scenario where the operating system update is in a fully
+disconnected environment and you want to perform updates via e.g. inserting
+a USB drive, one can do this by copying the desired OS container image to
+e.g. an `oci` directory:
+
+```bash
+skopeo copy docker://quay.io/exampleos/myos:latest oci:/path/to/filesystem/myos.oci
+```
+
+Then once the USB device containing the `myos.oci` OCI directory is mounted
+on the target, use
+
+```bash
+bootc switch --transport oci /var/mnt/usb/myos.oci
+```
+
+The above command can only be invoked once currently; thereafter, use `bootc upgrade`
+as normal to fetch updates from the USB device.
+
+This process can all be automated by creating systemd
+units that look for a USB device with a specific label, mount (optionally with LUKS
+for example), and then trigger the bootc upgrade.