readme: add 3rd party client: milahu/vpn-client-pia-wireguard-posix-shell

[milahu]

add my client vpn-client-pia-wireguard-posix-shell to readme

using this on my laptop, which is running 24/7 ... no crashes so far : )

could be slightly useful for writing an openwrt backend,
since openwrt has only sh, no bash

... but wg-quick is written in bash.
there is wg-quick.posix.sh by Rowan Thorpe (ported from wg-quick),
but 1. its buggy, and 2. openwrt has the uci interface for editing the config (iptables, etc)

[g00nix]

I will read through this before adding it to the readme. Looks really cool at the first glance.

[milahu]

looks good on paper ; )

there is a bug in wg-quick.posix.sh
so currently, this is not a pure posix-shell client, but still needs bash

[g00nix]

@milahu I started reading and I found this. Do you think you can find a better solution?

[milahu]

that's dead code in the if false; then ... fi block

its a draft for openWRT backend, using uci commands
of course, the config vars like network.pia.private_key should be in the config file

the actual code is using config_file and default_config_file to store the config

[g00nix]

Private keys should not be present anywhere in the repo and should always be generated. Even if you don't use that private key and it's just there as an example, it sill looks weird.