README.md: document --progress better

This commit adds the missing documentation for `--progress` in the
README. Not having this lead to some confusion, e.g. in questions
in osbuild#765 that
a better README would have avoided. Hopefully this answers the
questions :)

README.md

@@ -128,6 +128,7 @@ Usage:
 Flags:
       --chown string           chown the ouput directory to match the specified UID:GID
       --tls-verify             require HTTPS and verify certificates when contacting registries (default true)
+      --progress string        type of progress bar to use (e.g. plain,term)
       --type string            image type to build [qcow2, ami] (default "qcow2")
       --target-arch string     architecture to build image for (default is the native architecture)
 ```
@@ -137,6 +138,7 @@ Flags:
 | Argument          | Description                                                                                               | Default Value |
 |-------------------|-----------------------------------------------------------------------------------------------------------|:-------------:|
 | **--chown**       | chown the output directory to match the specified UID:GID                                                 |       ❌      |
+| **--progress**   | Show progress in the given format, supported: plain,term,debug. If empty it is auto-detected               |       ❌      |
 | **--rootfs**      | Root filesystem type. Overrides the default from the source container. Supported values: ext4, xfs, btrfs |       ❌      |
 | **--tls-verify**  | Require HTTPS and verify certificates when contacting registries                                          |    `true`     |
 | **--type**        | [Image type](#-image-types) to build                                                                      |    `qcow2`    |
@@ -172,6 +174,16 @@ a non-native architecture bootc OCI image, say, building for x86_64 from an arm-
 `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.
 
+## Progress types
+
+The following progress types are supported:
+
+* plain: No spinners or progress bar, just information and full osbuild output
+* term: Terminal based output, spinner, progressbar and most details of osbuild are hidden
+* debug: Details how the progress is called, mostly useful for bugreports
+
+Note that when no value is given the progress is auto-detected baed on the environment. When `stdin` is a terminal the "term" progress is used, otherwise "plain". The output of `plain` is exactaly the same as it was before progress reporting was implemented.
+
 ## ☁️ Cloud uploaders
 
 ### Amazon Machine Images (AMIs)