Add upgrading docs explaining the config file change for logrotate

Author: julianbrost

doc/16-upgrading-icinga-2.md

@@ -8,6 +8,17 @@ Specific version upgrades are described below. Please note that version
 updates are incremental. An upgrade from v2.6 to v2.8 requires to
 follow the instructions for v2.7 too.
 
+## Upgrading to v2.15.1, v2.14.7, or v2.13.13 <a id="upgrading-to-2-15"></a>
+
+These tree versions include the same fix to the configuration for logrotate. As this file is tracked as a configuration
+file by package managers, it may not be updated automatically if that file was modified locally. After upgrading, make
+sure to check if there are any files with an extension like `.dpkg-dist` or `.rpmnew` next to it. If so, you need to
+incorporate the changes into your configuration manually.
+
+To verify that the fix was applied correctly, check the contents of `/etc/logrotate.d/icinga2`: If the file uses the
+command `"$DAEMON" internal signal --sig SIGHUP --pid "$pid"` (instead of `kill -HUP "$pid"`), it was upgraded
+correctly.
+
 ## Upgrading to v2.15 <a id="upgrading-to-2-15"></a>
 
 ### Icinga DB <a id="upgrading-to-2-15-icingadb"></a>