Upgrading docs for v1.1.1

doc/04-Upgrading.md

@@ -3,6 +3,22 @@
 Specific version upgrades are described below. Please note that version upgrades are incremental.
 If you are upgrading across multiple versions, make sure to follow the steps for each of them.
 
+## Upgrading to Icinga DB v1.1.1
+
+Please apply the `1.1.1.sql` upgrade script to your database.
+For package installations, you can find this file at `/usr/share/doc/icingadb/schema/mysql/upgrades/` or
+`/usr/share/doc/icingadb/schema/pgsql/upgrades/`, depending on your database type.
+
+Note that this upgrade will change the `history` table, which can take some time depending on the size of the table and
+the performance of the database. While the upgrade is running, that table will be locked and can't be accessed. This
+means that the existing history can't be viewed in Icinga Web and new history entries will be buffered in Redis.
+
+As the daemon checks the schema version, the recommended way to perform the upgrade is to stop the daemon, apply the
+schema upgrade and then start the new daemon version. If you want to minimize downtime as much as possible, it is safe
+to apply this schema upgrade while the Icinga DB v1.1.0 daemon is still running and then restart the daemon with the
+new version. Please keep in mind that depending on the distribution, your package manager may automatically attempt to
+restart the daemon when upgrading the package.
+
 ## Upgrading to Icinga DB v1.0
 
 **Requirements**