fts: Document 3rd party plugins

Author: cmouse

docs/core/plugins/fts.md

@@ -43,6 +43,14 @@ Dovecot maintains these FTS indexing engines:
 | [[plugin,fts_solr]] | Interface to [Apache Solr](https://solr.apache.org/); stores data remotely. |
 | [[plugin,fts_flatcurve]] | [Xapian](https://xapian.org/) based driver; stores data locally. |
 
+There are also 3rd party provided FTS plugins:
+
+| Name | Description |
+| ---- | ----------- |
+| fts-xapian | https://github.com/grosjo/fts-xapian |
+
+If you want your plugin here, please open pull request to [dovecot/documentation](https://github.com/dovecot/documentation/pulls/new) repository.
+
 ## Searching In Dovecot
 
 When a FTS indexing driver is not present, searches use a slow sequential