In README, no dependency help commands for linux

In [README](https://github.com/CodeChain-io/codechain-indexer/#create-the-database-and-user-on-the-postgresql), the commands for installing dependencies are only for macOS currently.

Please add commands for linux.
such as
`sudo apt-get install postgresql-11`
`/usr/lib/postgresql/11/bin/pg_ctl -D /var/lib/postgresql/11/main -l logfile start`
`sudo -u postgres psql -f create_user_and_db.sql`