Add elasticsearch plugin installation guide (#1614)

- fix link

README.textile

@@ -113,7 +113,10 @@ to use the "staging.lobid.org"-domain and thus the staging-_context.jsonld_ will
 
 h3. Elasticsearch index
 
-See https://github.com/hbz/lobid-resources/wiki/Maintaining-Alma.lobid-API.
+We use the plugin "org.xbib.elasticsearch:elasticsearch-plugin-bundle:5.4.1.0":https://github.com/jprante/elasticsearch-plugin-bundle#elasticsearch-5x
+Follow the "installation guide for this plugin.":https://github.com/hbz/lobid-resources/issues/1615#issuecomment-1516331254
+
+Have a look at the "maintaining guide.":https://github.com/hbz/lobid-resources/wiki/Maintaining-lobid-API
 
 h1. License