docs: Describe offline updates with static deltas

This one isn't immediately obvious that it's possible.

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

docs/copying-deltas.md

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+---
+nav_order: 101
+---
+
+# Static deltas for offline updates
+{: .no_toc }
+
+1. TOC
+{:toc}
+
+<!-- SPDX-License-Identifier: (CC-BY-SA-3.0 OR GFDL-1.3-or-later) -->
+
+OSTree supports generating "self-contained" static delta files, via an
+invocation similar to the following:
+
+```
+$ ostree --repo=/path/to/repo static-delta generate --min-fallback-size=0 --filename=delta-update-file --from=<from> <to>
+```
+
+Note the usage of `--min-fallback-size=0` to ensure that the delta is self-contained.
+
+Then, you can copy `delta-update-file` to a USB key or similar, and a target system can apply it via e.g.:
+
+```
+$ ostree --repo=/ostree/repo static-delta apply-offline /path/to/delta-update-file
+```
+
+The above invocation will merely apply the content into the repository.
+To make it bootable, this will then need to be further followed up by e.g. `ostree admin deploy <to>` or
+with a higher level tool such as `rpm-ostree`, via `rpm-ostree deploy :<to>`.
+
+