Doc: Add note that only AWS S3 is supported (#223)

Clarify docs that no other S3 compatible storage solutions are supported.

Author: ppf2

docs/index.asciidoc

@@ -23,6 +23,8 @@ include::{include_path}/plugin_header.asciidoc[]
 
 This plugin batches and uploads logstash events into Amazon Simple Storage Service (Amazon S3).
 
+IMPORTANT: The S3 output plugin only supports AWS S3. Other S3 compatible storage solutions are not supported.
+
 S3 outputs create temporary files into the OS' temporary directory. 
 You can specify where to save them using the `temporary_directory` option.