Update readme on how to create new hosts

[pinpox]

The Creating new Hosts section was written before the use of flakes. New instructions should be written.
The old documentation for reference:

# Creating new Hosts. [TODO, this section is outdated!]

The following describes how to create new hosts to be included in this project
structure. It assumes a working NixOS installation on a new machine. The
following steps further assume you are logged in as root (e.g. via SSH)

## Preliminary Checks

- Check that hostname is set
- Check machine is connected to the internet
- Check timezone is correct
- Check nix-channel is correct

## Create Secrets

The following will create a new set of keys to be added to the `/secrets`
directory of this host.

```bash
# Create SSH keys
ssh-keygen -t ed25519 -f /secrets/$(hostname)/ssh/id_ed25519

# Create wireguard keys
# Use if `wireguard` is not installed: nix-shell -p pkgs.wireguard
wg genkey > /secrets/$(hostname)/wireguard/privatekey
wg pubkey < /secrets/$(hostname)/wireguard/privatekey > /secrets/$(hostname)/wireguard/publickey

# Create borg passphrase
# Use if `pwgen` is not installed: nix-shell -p pkgs.pwgen
pwgen 20 > /secrets/$(hostname)/borg/repo-passphrase

TODO add to pass