Add a README

Signed-off-by: Stéphane Graber <stgraber@stgraber.org>

README.md

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+# Introduction
+Incus OS is a minimal immutable OS image dedicated to running [Incus](https://linuxcontainers.org/incus).
+It's based on [Debian](https://www.debian.org) Testing and built using [mkosi](https://github.com/systemd/mkosi).
+
+This aims at provided a very fast, safe and reliable way to run an Incus server.
+
+# Security features
+Incus OS is designed to run on systems using UEFI with Secure Boot enabled.
+On first boot, it will automatically add the relevant Secure Boot key
+(requires the system be in setup mode).
+
+This ensures that only our signed image can be booted on the system.
+The image then uses dm-verity to validate every bit that's read from disk.
+
+All throughout boot, artifacts get measured through the TPM with the TPM
+state used to manage disk encryption.
+
+This effectively ensures that the system can only boot valid Incus OS
+images, that nothing can be altered on the system and that any
+re-configuration of the system requires the use of a recovery key to
+access the persistent storage.
+
+When updating, Incus OS uses an A/B update mechanism to reboot onto the
+newer version while keeping the previous version available should a
+revert be needed.
+
+# Status
+Incus OS is still in early development, the instructions below are there
+to help try it out, mostly for testing purposes as new features get
+added.
+
+# Testing
+Currently all development and testing of Incus OS is done through Incus VMs.
+The instructions below assume a functional Incus environment with VM support.
+
+## Using the Github releases
+Two scripts are available to test Incus OS using the publicly published releases.
+
+Creating a new Incus OS VM can be done with:
+
+    ./scripts/spawn-image VERSION NAME
+
+This will retrieve the relevant image from Github and create a VM using it.
+It will also automatically load the relevant packages (`incus` and `debug`).
+
+To update it to a newer version, you can do:
+
+    ./scripts/update-image VERSION NAME
+
+This will retrieve the update files and apply the update.
+
+## By building your own images
+Building your own images require the current version of `mkosi`.
+
+To build an image, run:
+
+    make
+
+To load that image as a VM, run:
+
+    make test
+
+To load the packages, run:
+
+    make test-extensions
+
+To test an update, build a new image and update to it with:
+
+    make
+    make update