README: document the new --extra-artifacts=sbom option

Small commit that tweaks the README to include this new option.

.spellcheck-en-custom.txt

@@ -14,4 +14,7 @@ qcow
 UEFI
 datadir
 copr
+SBOM
+SBOMs
+spdx
 toml

README.md

@@ -42,6 +42,8 @@ $ sudo dnf install osbuild osbuild-depsolve-dnf
 
 ## Examples
 
+### Listing
+
 To see the list of buildable images run:
 ```console
 $ image-builder list-images
@@ -52,6 +54,8 @@ rhel-10.0 type:ami arch:x86_64
 ...
 ```
 
+### Building
+
 To actually build an image run:
 ```console
 $ sudo image-builder build qcow2 --distro centos-9
@@ -60,6 +64,8 @@ $ sudo image-builder build qcow2 --distro centos-9
 this will create a directory `centos-9-qcow2-x86_64` under which the
 output is stored.
 
+### Blueprints
+
 Blueprints are supported, first create a `config.toml` and put e.g.
 the following content in:
 ```toml
@@ -80,6 +86,14 @@ $ sudo image-builder build qcow2 --blueprint ./config.toml --distro centos-9
 ...
 ```
 
+### SBOMs
+
+It is possible to generate spdx based SBOM (software bill of materials)
+documents as part of the build. Just pass `--extra-artifacts=sbom` and
+it will put them into the output directory.
+
+### Filtering
+
 When listing images, it is possible to filter:
 ```console
 $ image-builder list-images --filter ami
@@ -105,6 +119,8 @@ The following filters are currently supported, shell-style globbing is supported
  * type: the image type name (e.g. qcow2)
  * bootmode: the bootmode (legacy, UEFI, hybrid)
 
+### Output control
+
 The output can also be switched, supported are "text", "json":
 ```console
 $ image-builder list-images --output=json