Added a readme file

persistence/README.md

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+## Database Persistence ##
+The persistence package includes classes to store different class objects in a Postgres database.
+
+### models ###
+This defines models for all class objects that we want to persist. It is used for Object Relation Mapping (ORM)
+of the class objects to database table columns
+
+### repositories ###
+This defines repository classes that contain CRUD methods for database operations.
+
+### config_db ##
+This Python file is a configuration class that contains variables that map to configuration parameters in a .env file. 
+It also contains a method ``def conn_string()`` which returns the connection string to a Postgres database.
+
+### Base ##
+This class has a constructor that establishes a database connection and returns a reference for database-related CRUD operations.
+
+### db_migration ###
+This Python file is in the root of Hub and should be run to create all the required Postgres database tables
+
+### Database Configuration Parameter ###
+A .env file (or environment variables) with configuration parameters described below are needed to establish a database connection:
+```
+DB_NAME=postgres-database-name
+DB_USER=postgres-database-user
+DB_PASSWORD=postgres-database-password
+DB_HOST=database-host
+DB_PORT=database-port
+```