[DOCS] Adding ILM settings file

Author: debadair

docs/reference/settings/ilm-settings.asciidoc

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+[role="xpack"]
+[[ilm-settings]]
+=== {ilm-cap} settings
+
+These index-level {ilm-init} settings are typically configured through index
+templates. For more information, see <<ilm-gs-create-policy>>.
+
+`index.lifecycle.name`::
+The name of the policy to use to manage the index.
+
+`index.lifecycle.rollover_alias`::
+The index alias to update when the index rolls over. Required if the specified
+policy contains a rollover action. When the index rolls over, the alias is
+updated to reflect that the index is no longer the
+<<aliases-write-index, write index>>.