docs/usage: New doc

There is some overlap with `install.md`, but that doc is all about
how to just get started.

This doc aims to cover things that are somewhat different about
the base image we ship here versus just what one might get
from installing the same RPM packages.

The automatic updates being on by default is a big one.

Signed-off-by: Colin Walters <walters@verbum.org>

docs/usage.md

@@ -0,0 +1,37 @@
+---
+nav_order: 3
+---
+
+# Operating system content and usage
+
+## Automatic updates enabled by default
+
+The base image here enables the
+[bootc-fetch-apply-updates.service](https://github.com/containers/bootc/blob/main/manpages-md-extra/bootc-fetch-apply-updates.service.md)
+systemd unit which automatically finds updated container images from the
+registry and will reboot into them.
+
+### Controlling automatic updates
+
+First, one can disable the timer entirely as part of a container build:
+
+```dockerfile
+RUN systemctl mask bootc-fetch-apply-updates.timer
+```
+
+Alternatively, one can use systemd "drop-ins" to override the timer
+(for example, to schedule updates for once a week), create a file
+like this, named e.g. `50-weekly.conf`:
+
+```systemd
+[Timer]
+# Clear previous timers
+OnBootSec= OnBootSec=1w OnUnitInactiveSec=1w
+```
+
+Then add it into your container:
+
+```dockerfile
+RUN mkdir -p /usr/lib/systemd/system/bootc-fetch-apply-updates.timer.d
+COPY 50-weekly.conf /usr/lib/systemd/system/bootc-fetch-apply-updates.timer.d
+```