docs: README: how to build the Steamify ISO yourself (podman)

README.md

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+# Steamify CachyOS ISO for the Steam Machine
+
+A CachyOS desktop ISO for the Valve Steam Machine with
+[Steamify](https://github.com/theupriser/steamify-cachyos) built in:
+- The live session has the Steam Machine drivers (power-off fix).
+- The installer ("CachyOS with Steamify Installer") has a Steamify page where
+  you pick the options and whether the PC starts in gaming mode or on the
+  desktop. Steamify is applied during the install.
+
+## Building the ISO yourself
+
+You need about 20 GB of free space, and a Linux PC with
+[podman](https://podman.io) (or Docker: replace `podman` with `docker`). The
+build runs in CachyOS's own container, so the PC doesn't have to run CachyOS.
+
+```bash
+git clone -b feat/steamify https://github.com/theupriser/steammachine-cachyos-live-iso
+cd steammachine-cachyos-live-iso
+
+# Put Steamify on the ISO: the newest release. To use your own checkout,
+# pass its path: ./steamify-prepare.sh ~/steamify-cachyos
+./steamify-prepare.sh
+
+mkdir -p ~/iso-cache    # downloaded packages, kept for the next build
+sudo podman run --rm -t --privileged --network=host \
+  --pids-limit=-1 --ulimit nofile=65536:65536 \
+  -v ~/iso-cache:/var/cache/pacman/pkg -v "$PWD":/iso -w /iso \
+  docker.io/cachyos/cachyos:latest bash -c '
+    pacman-key --init && pacman-key --populate &&
+    pacman -Syu --noconfirm --needed archiso mkinitcpio-archiso git squashfs-tools grub sudo &&
+    ./build-live-modules.sh && ./build-calamares-modules.sh &&
+    ./buildiso.sh -p desktop -w'
+```
+
+The ISO is written to `out/desktop/steamify-cachyos-<date>.iso`. Write it to
+a USB stick (for example with `dd` or Fedora Media Writer) and boot the Steam
+Machine from it.
+
+**Why these steps:**
+- `build-live-modules.sh` builds the power-off fix for the ISO's kernels.
+  Without it, the Steam Machine starts up again right after shutting down.
+- `build-calamares-modules.sh` builds the installer module that shows the
+  Steamify page. CachyOS's installer package leaves it out.
+- `--pids-limit=-1` and `--ulimit nofile=...` are needed because pacman runs
+  out of processes or open files in the container otherwise (GPGME errors).
+- `--network=host` gives the container DNS.
+- `-w` removes the build folder afterwards. Before building again, run
+  `sudo rm -rf build out`.
+
+To build on CachyOS directly, without a container, install the same packages
+with pacman and run the three scripts with `sudo`.
+
+---
+
+The notes below are from CachyOS's original repository.
+
 These are the basic needed files and folders to build CachyOS system.
 
 ### buildiso