Add a known issue for upgrading from 5.x to 6.3.0
This is due to
[[release-notes-6.3.0]]
== {es} version 6.3.0
+[float]
+=== Known issues
+
+Upgrades from any 5.x version will fail for indexes which are prepared using the <<indices-synced-flush, _synced flush API>>,
+or were automatically sync-flushed due to inactivity. If upgrading from those versions, please
+wait for 6.3.1 to be released.
+
+Clusters with a Gold or Platinum license that are upgrading to 6.3 will need to explicitly set
+`xpack.security.enabled: true` in `elasticsearch.yml` to upgrade successfully.
+If this value is not set, the cluster will be unable to form after upgrade.
+
Also see <<breaking-changes-6.3>>.
[[breaking-6.3.0]]
