mod: ReadMe file according to latest docker compose version

[TheSpiritMan]

Hello Team

In today's docker engine:1.48, compose sub-command is already available. So we do not need to install docker-compose binary differently.

In this commit, I have done:

• Modified Readme.md to use docker compose command instead of docker-compose.
• Replace <db_container_name> with the actual db name i.e reverse-hash-service-db-1.
• Remove # Create rhs db section because rhs database is initially created from docker-compose.yml file.
• Added .gitignore file to add .postgres-data/ in git ignore list.

Note: I have tested this in Ubuntu System using docker engine version 1.48.