Add configuration for blob storage persistence

[yoh1496]

To keep blob data after stopping containers,
add definition of volumes in personium service.

[yoh1496]

How about append below lines to docker-compose.yml

# in personium service
  volumes:
    - personium-data:/personium_nfs/personium-core

# in volumes section
  personium-data:
    driver: local

[yoh1496]

Write to README.md