add doc on multi-arch builds

Signed-off-by: sallyom <[email protected]>

README.md

@@ -133,20 +133,22 @@ Usage:
     <imgref>
 
 Flags:
-      --chown string    chown the ouput directory to match the specified UID:GID
-      --config string   build config file
-      --tls-verify      require HTTPS and verify certificates when contacting registries (default true)
-      --type string     image type to build [qcow2, ami] (default "qcow2")
+      --chown string           chown the ouput directory to match the specified UID:GID
+      --config string          build config file
+      --tls-verify             require HTTPS and verify certificates when contacting registries (default true)
+      --type string            image type to build [qcow2, ami] (default "qcow2")
+      --target-arch string     architecture to build image for (default is the native architecture)
 ```
 
 ### Detailed description of optional flags
 
-| Argument         | Description                                                      | Default Value |
-|------------------|------------------------------------------------------------------|:-------------:|
-| **--chown**      | chown the ouput directory to match the specified UID:GID         |       ❌      |
-| **--config**     | Path to a [build config](#-build-config)                         |       ❌      |
-| **--tls-verify** | Require HTTPS and verify certificates when contacting registries |    `true`     |
-| **--type**       | [Image type](#-image-types) to build                             |    `qcow2`    |
+| Argument          | Description                                                         | Default Value |
+|-------------------|---------------------------------------------------------------------|:-------------:|
+| **--chown**       | chown the ouput directory to match the specified UID:GID            |       ❌      |
+| **--config**      | Path to a [build config](#-build-config)                            |       ❌      |
+| **--tls-verify**  | Require HTTPS and verify certificates when contacting registries    |    `true`     |
+| **--type**        | [Image type](#-image-types) to build                                |    `qcow2`    |
+| **--target-arch** | [Target arch](#-target-architecture) to build                       |       ❌      |
 
 The `--type` parameter can be given multiple times and multiple outputs will
 be produced.
@@ -165,6 +167,17 @@ The following image types are currently available via the `--type` argument:
 | `anaconda-iso`        | An unattended Anaconda installer that installs to the first disk found.               |
 | `raw`                 | Unformatted [raw disk](https://en.wikipedia.org/wiki/Rawdisk).                        |
 
+## 💾 Target architecture
+
+Specify the target architecture of the system on which the disk image will be installed on. By default,
+`bootc-image-builder` will build for the native host architecture. The target architecture
+must match an available architecture of the `bootc-image-builder` image you are using to build the disk image.
+Currently, `amd64` and `arm64` are included in `quay.io/centos-bootc/bootc-image-builder` manifest list.
+The architecture of the bootc OCI image and the bootc-image-builder image must match. For example, when building
+a non-native architecture bootc OCI image, say, building for x86_64 from an arm-based Mac, it is possible to run
+`podman build` with the `--platform linux/amd64` flag. In this case, to then build a disk image from the same arm-based Mac,
+you should provide `--target-arch amd64` when running the `bootc-image-builder` command.
+
 ## ☁️ Cloud uploaders
 
 ### Amazon Machine Images (AMIs)