docs: add readme

README.md

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+# Flatcar Proxmox VE cloud image
+
+This repository contains a customized OEM partition for Flatcar that supports Proxmox VE.
+
+## Supported features
+
+- Set hostname
+- Write SSH keys
+- Write systemd networkd configuration
+- Write attributes file (metadata)
+
+## How it works
+
+The following is run on Github Actions :
+
+- Download the latest Flatcar image (stable)
+- Mount the OEM partition
+- Write the contents of the `oem/` directory into the OEM partition
+- Download and include our [forked Afterburn](https://github.com/coreos/afterburn/pull/1023) that supports Proxmox VE
+- Bundle the image again and upload it to Github releases.
+
+## What's included in the OEM partition ?
+
+- Ignition configuration file that will write our custom files
+- Systemd configuration files that run our modifed version of Afterburn.
+- The forked afterburn binary