Update README.md

[MarcDorval]

Adding info on prerequisites (Google Cloud Integration)

[jbrian31]

Please change your YAML for your conditional card as its no longer working, this is a better working version.

type: conditional
conditions:

• condition: state
  entity: binary_sensor.backups_stale
  state_not: "off"
  card:
  type: markdown
  content: >-
  Backups are stale! Please visit the "Home Assistant Google Drive Backup"
  add-on
  status page for details.

  -Last Backup: {{state_attr('sensor.backup_state','last_backup')}}

  -Last Upload:{{state_attr('sensor.backup_state','last_uploaded')}}
  title: Stale Backups!