Update README.md with instructions to add user to docker group

[tylermilner]

When setting up my Internet Pi on my Raspberry Pi 5, I encountered the permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock error described in #620 when running ansible-playbook main.yml. Following the suggestion to add the Raspberry Pi user to the docker user group (via sudo usermod -aG docker $USER) fixed the issue and allowed ansible-playbook main.yml to run successfully.

This adds a note about this fix to the installation instructions in README.md. It might also be better to just add this as an explicit step in the setup process if everyone will need to be doing this now.