blueprint-reference: add Ignition config section

Signed-off-by: Irene Diez <[email protected]>

osbuild-composer/src/blueprint-reference/blueprint-reference.md

@@ -227,6 +227,39 @@ The service names are systemd service units. You may specify any systemd unit fi
 enabled = ["sshd", "cockpit.socket", "httpd"]
 disabled = ["postfix", "telnetd"]
 ```
+
+### Ignition
+
+The `customizations.ignition` section allows to provide
+[Ignition](https://coreos.github.io/ignition/) configuration files to be used
+in `edge-simplified-installer` ISO images. Check the `r4e`
+[butane](https://coreos.github.io/butane/specs/) specification for a
+description of the supported configuration options.
+
+The blueprint configuration can be done *either* by embedding ignition
+configuration file into the ISO (this can be done *either* by providing a
+Base64 encoded Ignition file or a URL to where the Ignition file configurarion
+lives), or providing a provisioning URL that will be fetched at first boot.
+
+The two possible embedded configuration options are the following:
+```toml
+[customizations.ignition.embedded]
+ignition_data = "eyJ --- BASE64 STRING TRIMMED --- 19fQo="
+```
+
+```toml
+[customizations.ignition.embedded]
+url = "http://some-server/configuration.ig"
+```
+
+The way of specifiying an URL with an ignition file to be fetched at first boot
+is the following:
+
+```toml
+[customizations.ignition.firstboot]
+url = "http://some-server/configuration.ig"
+```
+
 ## Distribution selection with blueprints
 
 The blueprint now supports a new `distro` field that will be used to select the