readme: add instructions for a manual update

The readme now includes instructions for using the provided
serve-package to apply a manual update.

README.md

@@ -23,3 +23,50 @@ These differences include:
    As much as possible the code includes comments about these extensions.
 
  - Many data fields not used by CoreUpdate are omitted.
+ 
+## `serve-package`
+
+This project includes a very simple program designed to serve a single Container Linux package on the local host. It is intended to be used as a manual updater for a machine that is not able to use a full-fledged CoreUpdate instance. Binaries are available for each released version on the [releases page](https://github.com/coreos/go-omaha/releases). `serve-package` can also be built from source using the provided Makefile:
+
+```bash
+make
+```
+
+The binary will be available in the `bin/` folder.
+
+It is recommended that the server be run directly on the machine you intend to update. Go to the [Container Linux release notes](https://coreos.com/releases/) and find the version number for the release you would like to update to. The update payload can be retrieved from
+
+```
+https://update.release.core-os.net/amd64-usr/<version>/update.gz
+```
+
+where `<version>` is the version number you retrieved from the releases page. For example, `https://update.release.core-os.net/amd64-usr/1576.4.0/update.gz` is the payload required to update to Container Linux version 1576.4.0.
+
+Copy the update payload and the `serve-package` binary to the server you would like to update. `serve-package` can be run as follows:
+
+```bash
+./serve-package --package-file update.gz --package-version <version>
+```
+
+By default, the server listens on `localhost:8000`. This can be modified using the `--listen-address` option.
+
+Next, `update_engine` needs to be configured to use the local server that was just set up:
+
+```bash
+echo "SERVER=http://localhost:8000/v1/update" | sudo tee -a /etc/coreos/update.conf
+```
+
+Restart `update_engine` and tell it to check for an update:
+
+```bash
+sudo systemctl restart update-engine.service
+update_engine_client -check_for_update
+```
+
+If `locksmithd.service` is running, the machine will restart once it has updated to the latest version. Otherwise, watch the logs from `update-engine.service` to determine when the update is complete and the machine is ready to restart:
+
+```bash
+journalctl -u update-engine.service -f
+# wait for a line that says "Update successfully applied, waiting for reboot"
+sudo systemctl reboot
+```