Update README for swupdate

Signed-off-by: Quirin Gylstorff <quirin.gylstorff@siemens.com>

README.md

@@ -31,6 +31,12 @@ Using 3rd-party mirrors, for example
 ./kas-container build kas-iot2050-example.yml:kas/opt/mirror-example.yml
 ```
 
+Using SWUpdate with a A/B rootfs configuration, for example
+
+```shell
+./kas-container build kas-iot2050-example.yml:kas/opt/swupdate-example.yml
+```
+
 After build complete, the final image is under
 
 ```text
@@ -145,3 +151,60 @@ device.
 
 NOTE: This selection is not persistent. The boot loader will fall back to its
 default boot order after reset.
+
+## Building with SWUpdate support
+
+The option `kas/opt/swupdate-example.yml` will create an image  with a
+[double copy root file system](https://sbabic.github.io/swupdate/overview.html#double-copy-with-fall-back)
+You can find the image under
+`build/tmp/deploy/images/iot2050/iot2050-image-swu-example-iot2050-debian-iot2050.wic.img`
+
+NOTE: The image has a size of 7 Gigabytes.
+
+It also will create a binary for updating the system at
+This call will write the image to the unused root partition`build/tmp/deploy/images/iot2050/iot2050-image-swu-examp
+le-iot2050-debian-iot2050.swu`
+
+The binary can be uploaded to the device and be applied by executing:
+
+```shell
+$ swupdate -i iot2050-image-swu-example-iot2050-debian-iot2050.swu
+```
+SWUpdate will write the image to the unused root partition and
+sets the necessary U-Boot variables.
+
+After a reboot, the device boots into the new root file system.  If the boot is
+successful the update process needs to be completed by calling:
+
+```
+$ confirm_update.sh success
+```
+
+The script sets the update state in the U-Boot environment to the initial state.
+
+If the update is not successful, a reboot will roll back to the previous root file system.
+This rollback is confirmed by calling:
+
+```
+$ confirm_update.sh failed
+```
+
+This call reverts the U-Boot environment to the initial state.
+
+### U-Boot environment
+
+The bootloader environment needs to be adapted to select the correct
+root file system during boot. This adaptation occurs during the first
+boot by executing the `patch-u-boot-env.service`. This systemd-service writes the necessary variables to the
+U-Boot environment. After writing the U-Boot environment, an update with SWUpdate is possible.
+
+#### Revert to the default environment
+
+If it is necessary to revert to the default U-Boot environment, reset the device and
+interrupt the boot when it counts down ("Hit any key to stop autoboot").
+Then type
+
+```shell
+=> env default -a -f
+=> saveenv
+```