Add some notes regarding direct_path_prefix_object

README.md

@@ -12,6 +12,8 @@
 
 - **path_prefix** prefix of target keys (string, required)
 
+- **direct_path_prefix_object** flag to enable treating `path_prefix` as a single object key (boolean, default to `false`). Since list-objects operation on S3 is eventually consistent, using this for a  more reliable way to retrieve a single object when you know the exact object key.
+
 - **endpoint** S3 endpoint login user name (string, optional)
 
 - **region** S3 region. endpoint will be in effect if you specify both of endpoint and region (string, optional)

embulk-input-s3/src/main/java/org/embulk/input/s3/AbstractS3FileInputPlugin.java

@@ -82,6 +82,12 @@ public interface PluginTask
         @ConfigDefault("false")
         public boolean getSkipGlacierObjects();
 
+        /**
+         * When this is on, "path_prefix" config will be treated as a single object key,
+         *
+         * Since list-objects operation on S3 is eventually consistent, using this for a
+         * more reliable way to retrieve a single object when you know the exact object key.
+         */
         @Config("direct_path_prefix_object")
         @ConfigDefault("false")
         public boolean getDirectPathPrefixObject();