Migrate dashboard positions data from v1 to v2 format (apache#5463)

* Migrate dashboard positions data from v1 to v2 format

* UPDATING.md

* rebase onto master

(cherry picked from commit fd2d4b0)

UPDATING.md

@@ -3,6 +3,20 @@
 This file documents any backwards-incompatible changes in Superset and
 assists people when migrating to a new version.
 
+## 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
+to automatically convert dashboard grid to nested layout data. To be safe, please 
+take a database backup prior to this upgrade. It's the only way people could go 
+back to a previous state.
+
+
+## Superset 0.26.0
+* Superset 0.26.0 deprecates the `superset worker` CLI, which is a simple
+wrapper around the `celery worker` command, forcing you into crafting
+your own native `celery worker` command. Your command should look something
+like `celery worker --app=superset.sql_lab:celery_app --pool=gevent -Ofair`
+
 ## Superset 0.25.0
 Superset 0.25.0 contains a backwards incompatible changes.
 If you run a production system you should schedule downtime for this