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`

Open the README's "Create the database and user on the PostgreSQL" section and compare the existing macOS dependency commands with the Linux commands listed in the issue. Add the Linux installation and setup commands, then confirm the README covers both platforms and the commands are formatted consistently.

Written by the indexing model from the issue text.