Add guide for using ILM with existing indices (elastic#43667)

In many cases, including migration from previous versions of data
shippers (e.g. Beats), it is useful to use ILM to manage historical
indices, which are no longer being written to. This commit adds a guide
which gives an example of how to do that.

Author: gwbrown

docs/reference/docs/reindex.asciidoc

@@ -252,7 +252,7 @@ POST _reindex
 // CONSOLE
 // TEST[setup:twitter]
 
-
+[[reindex-scripts]]
 Like `_update_by_query`, `_reindex` supports a script that modifies the
 document. Unlike `_update_by_query`, the script is allowed to modify the
 document's metadata. This example bumps the version of the source document: