Create ER diagram and SQL Table for `Order` Service

[sokimaaa]

Context

• We are going to follow Database-Per-Service microservice pattern
• Order service is going to use PostgreSQL database
• We need to write ER (Entity Relationship) Diagrams to have visibility of database structure
• We need to have a script to run to instantiate all Tables

Acceptance Criteria

• The submodule db/postgres is created
• The init tables script is present and tested
• The ER diagram is present in 1-documentation/...
• The ER diagram is created using PlantUML
• Append README.md with docker command to run postgres db in container, e.g. docker run ....

Technical Details

• Explore the Database-Per-Service microservice pattern
• Explore what is ER Diagram
• Explore how to write ER Diagram using PlantUML
• Explore how to instantiate Postgres table
• Remember that Postgres is relational db