Document `package` format

Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>

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+# A container package
+
+Imagebuilder takes multiple `package` containers as input and produces a
+runnable u-boot script or a runnable disk image as output. This document
+describes the package format.
+
+A package is a container containing all the relevant binaries for a
+given component under the following path:
+
+```
+/home/builder/output-<name>/
+```
+
+For instance, for DomUs a package containes the kernel, rootfs,
+additional configuration. As an example, zynqmp-dom1-package contains
+the following files:
+
+```
+/home/builder/output-zynqmp-dom1/Image-domU
+/home/builder/output-zynqmp-dom1/domU-ramdisk.cpio
+/home/builder/output-zynqmp-dom1/passthrough-example-part.dtb
+```
+
+Which are the kernel, ramdisk, passthrough configuration respectively.