Add migration note on kernel parameters on install

This commit adds a note to the migration docs regarding how to avoid the
installation packages from setting kernel parameters on install.

docs/reference/migration/migrate_5_5.asciidoc

@@ -7,4 +7,15 @@
 
 Previous versions of Elasticsearch would skip hidden files and directories when
 scanning the plugins folder. This allows broken plugin installs to remain
-undetected so this leniency has been removed.
+undetected so this leniency has been removed.
+
+[[breaking_54_packaging_changes]]
+[float]
+=== Skip setting kernel parameters
+
+Elasticsearch attempts to set the kernel parameter `vm.max_map_count` during
+installation. In some environments this can fail. Previous versions of
+Elasticsearch accepted a flag `ES_SKIP_SET_KERNEL_PARAMTERS` to skip this
+writing. Starting in Elasticsearch 5.5.0, this flag has been removed and
+instead the same functionality can be achieved by masking the `systemd-sysctl`
+service.