Add instructions for host in README.md

[bitwiseguy]

I was having issues connecting to the postgres container because I left the host URL as localhost. Consider adding to the setup instructions to give the user direction that the URL should be the postgres container name.

Database Info:
Hostname: pg
Host: (this should be the postgres container name, i.e. `devp2p-network_pg_1`, not `localhost`)
Port: 5432
Username: pguser
Password: 3th3r3um
Database Name: devp2p