Mention database migration in README

[Glutexo]

Before starting using the application it is necessary to run the database migrations. Otherwise there will be no hosts table and requests will fail.

Mention this in the README and provide a guide on how to initialize the database. This includes:

• Create the database. This is probably done automatically when using Docker Compose.
• Run the migrations with python manage.py upgrade. Don’t forget to set the INVENTORY_DB_NAME and other environment variables.
• Mention that if using different a database for testing, the migration must be run for it too.