Update README.md (#630)

Add note about adding user to `docker` user group to fix permission denied error when running playbook based on discussion in #620.

Author: tylermilner

README.md

@@ -52,7 +52,7 @@ It should also work with Ubuntu for Pi, or Arch Linux, but has not been tested o
      - `example.config.yml` to `config.yml`
   5. Run the playbook: `ansible-playbook main.yml`
 
-> **If running locally on the Pi**: You may encounter an error like "Error while fetching server API version" or "connect: permission denied". If you do, please either reboot or log out and log back in, then run the playbook again.
+> **If running locally on the Pi**: You may encounter an error like "Error while fetching server API version" or "connect: permission denied". If you do, please either reboot or log out and log back in, then run the playbook again. If you continue to receive a "permission denied while trying to connect to the Docker daemon socket" error, then you may need to first add your user to the `docker` user group: `sudo usermod -aG docker $USER`, as outlined in the [Docker documentation](https://docs.docker.com/engine/install/linux-postinstall/).
 
 ## Usage