Docs: Added new entry for the SIREn plugin.

Closes #6961

docs/reference/modules/plugins.asciidoc

@@ -304,4 +304,5 @@ You can disable that check using `plugins.check_lucene: false`.
 * https://github.com/wikimedia/search-highlighter[Elasticsearch Experimental Highlighter] (by Wikimedia Foundation/Nik Everett)
 * https://github.com/salyh/elasticsearch-security-plugin[Elasticsearch Security Plugin] (by Hendrik Saly)
 * https://github.com/codelibs/elasticsearch-taste[Elasticsearch Taste Plugin] (by CodeLibs Project)
+* http://siren.solutions/siren/downloads/[Elasticsearch SIREn Plugin]: Nested data search (by SIREn Solutions)