Add sample script to connect a node to coralnet

[ethanfrey]

Just a simple shell script that people can easily read and use as the basis of more complex tools.
But also enough to get started quickly if you don't want to do more.

It can use docker images, and the scripts should accept env variables to set all URLs (to easily reconfigure for other testnets). This can be based on Ubuntu 18.04/20.04 or Debian 10 (buster) - pick one.
The following scripts are envisioned:

• Setup node
  • Install docker if not installed
  • corald init
  • download genesis file from github repo
  • configure seed nodes or persistent peers in config.toml (maybe just README to show how, not script)
• Run node in systemd
  • Set up a systemd service
  • enable and start service
  • add README how to view logs and restart
  • (note supervising docker in systemd is problematic: use https://github.com/ibuildthecloud/systemd-docker or alternative)