cmd/oci-image-tool: add README.md

Signed-off-by: Antonio Murdaca <runcom@redhat.com>

cmd/oci-image-tool/README.md

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+# oci-image-tool
+
+Simple command line tool to work with OCI images.
+
+## Usage
+```
+A tool for working with OCI images
+
+Usage:
+  oci-image-tool [command]
+
+Available Commands:
+  create-runtime-bundle Create an OCI image runtime bundle
+  unpack                Unpack an image or image source layout
+  validate              Validate one or more image files
+
+Use "oci-image-tool [command] --help" for more information about a command.
+```
+
+## Examples
+
+### create-runtime-bundle
+```
+$ skopeo copy docker://busybox oci:busybox-oci
+$ mkdir busybox-bundle
+$ ci-image-tool create-runtime-bundle --ref latest busybox-oci busybox-bundle
+$ cd busybox-bundle && sudo runc start busybox
+[...]
+```
+
+### unpack
+```
+$ skopeo copy docker://busybox oci:busybox-oci
+$ mkdir busybox-bundle
+$ oci-image-tool unpack --ref latest busybox-oci busybox-bundle
+tree busybox-bundle
+busybox-bundle
+├── bin
+│   ├── [
+│   ├── [[
+│   ├── acpid
+│   ├── addgroup
+│   ├── add-shell
+│   ├── adduser
+│   ├── adjtimex
+│   ├── ar
+│   ├── arp
+│   ├── arping
+│   ├── ash
+[...]
+```
+
+### validate
+```
+$ skopeo copy docker://busybox oci:busybox-oci
+$ oci-image-tool validate --type imageLayout --ref latest busybox-oci
+busybox-oci: OK
+```