Add S3 and HDFS repositories

docs/reference/modules/snapshots.asciidoc

@@ -85,6 +85,14 @@ point to the root of the shared filesystem repository. The following settings ar
 `concurrent_streams`:: Throttles the number of streams (per node) preforming snapshot operation. Defaults to `5`
 
 
+[float]
+===== Repository plugins
+
+Other repository backends are available in these official plugins:
+
+* https://github.com/elasticsearch/elasticsearch-cloud-aws#s3-repository[AWS Cloud Plugin] for S3 repositories
+* https://github.com/elasticsearch/elasticsearch-hdfs[HDFS Plugin]
+
 [float]
 === Snapshot