image: Add README

Add readme do document how to use ./image-builder/image_builder.sh

Signed-off-by: Jose Carlos Venegas Munoz <jose.carlos.venegas.munoz@intel.com>

image-builder/README.md

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+# Kata Containers image generation #
+
+A Kata Containers image is generated by the script `image_builder.sh`
+which uses a `rootfs` directory created by the script
+`rootfs-builder/rootfs.sh`.
+
+To create a guest OS image run:
+
+```
+$ ./image_builder.sh path/to/rootfs
+```
+
+Where `path/to/rootfs` is the directory pupulated by `rootfs.sh`. The
+script will check for following required binaries:
+
+- `/sbin/init` : The image must contain a `init` binary 
+- `/bin/kata-agent` : The image contain the Kata [agent]
+
+More information about usage:
+
+```
+$ ./image_builder.sh -h
+```
+
+[agent]: https://github.com/kata-containers/agent  "Kata agent"