add full documentation for new image tooling

[cyphar]

Currently all of our tooling only really has documentation for each piece. However, once #5 and #8 are merged we will (in principle) have all of the tooling necessary to create an OCI image from scratch using nothing but OCI tooling.

However, because of how each tool is split, we need to have documentation on how you can actually combine all of the pieces. While it may seem somewhat obvious once you've played with it for a bit, it would be quite useful for people who want to use the binaries (or even use the libraries) to understand what the flow should look like.

The documentation could just be an example shell session going from an empty directory to an OCI image which then can be unpacked and set up as a runtime bundle.