Add a note about upgrading to version 1.4.2.

Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>

UPGRADING.md

@@ -1,3 +1,23 @@
+To v1.4.2
+---------
+Upgrading to this version (especially from older version of Docker) can trigger a migration
+for existing containers when the AUFS file system was used to store Docker images on. Normally,
+this won't be a problem, as the migration is going to be performed automatically by Docker.
+
+Please note that there can be a momentary performance impact due to data being moved in place
+and new checksums being calculated to accommodate for the new format.
+
+Should the process of automatic migration have failed and/or latest version of Docker would
+refuse to start, then a manual removal of existing AUFS volume might be needed, before the
+Docker service can be brought back into an operational state. Please keep in mind that this
+would require images to be downloaded from the Docker Registry again, which might have an
+impact both on the underlying service availability and performance of the host while the
+containers are being downloaded.
+
+For more details about storage and Docker images, please see the following:
+
+  https://docs.docker.com/engine/userguide/storagedriver/imagesandcontainers/
+
 To v1.3.0
 ---------
 This version adds aufs support. Previously running docker containers will not be aware of this change and will need to be restarted manually.