message: UPDATING.md entry about dashboard v2 forced migration in 0.28
diff: This file documents any backwards-incompatible changes in Superset and
  assists people when migrating to a new version.
  +## Superset 0.28.0
  +* Superset 0.28 deprecates the previous dashboard layout. While 0.27
  + offered a migration workflow to users and allowed them to validate and
  + publish their migrated dashboards individually, 0.28 forces
  + the migration of all
  + dashboards through an automated db migration script. We
  + do recommend that you take a backup prior to this migration.
  +
  ## Superset 0.27.0
  * Superset 0.27 start to use nested layout for dashboard builder, which is not
  backward-compatible with earlier dashboard grid data. We provide migration script
  