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.

[github-actions]

This pr has been marked 'stale' due to lack of recent activity. If there is no further activity, the issue will be closed in another 30 days. Thank you for your contribution!

Please read this blog post to see the reasons why I mark issues as stale.

[tylermilner]

Looks like others have recently encountered the issue with the RPi user not being part of the docker user group in #637 and #640. Maybe it would be good to just make that part of the standard setup process. Otherwise, people will likely continue to encounter this problem and create additional GitHub issues.

[geerlingguy]

The logout/login or reboot should resolve that in most cases, but I'm fine adding a little extra since that way people can search the README and find it there.