update docs sse config usage

Author: anGie44

website/docs/r/s3_bucket.html.markdown

@@ -303,6 +303,11 @@ resource "aws_s3_bucket" "source" {
 }
 ```
 
+### Enable Default Server Side Encryption
+
+The `server_side_encryption_configuration` argument is read-only as of version 4.0.
+See the [`aws_s3_bucket_server_side_encryption_configuration` resource](s3_bucket_server_side_encryption_configuration.html.markdown) for configuration details.
+
 ### Using ACL policy grants
 
 ```terraform