Add basic systemd unit, flesh out deployment instructions

sudo apt install cryptsetup

```

Create a user account called `poolpay`. Equip it with the necessary permissions:

* Allow `poolpay` to access GPIO

* Allow `poolpay` to use `sudo` **to use `luks`** (don't give it general sudo access)!

Create a folder `/coli`, cd into it and make sure the permissions are correct.

Clone the repository (you can use a Project Access Token for that, Settings → Access Tokens) and cd into it.

```bash

cd /coli/poolpay

python3 -m venv venv

source venv/bin/activate

pip install -r requirements.txt

[Unit]

Description=PoolPay

[Service]

ExecStart=/coli/poolpay/venv/bin/python3 -m poolpay

WorkingDirectory=/coli/poolpay

User=poolpay

[Install]

WantedBy=multi-user.target