modify the README

README.md

@@ -19,6 +19,8 @@
 ## Example
 
 ```yaml
+exec:
+  min_output_tasks: 1
 in:
   type: filename 
   mulit_dir: ["../sample/sample_","../example/example_"]
@@ -26,6 +28,32 @@ in:
   order_by_modified_time: 1
   chunk_size: 1000
 ```
+Attention:
+exec:
+  min_output_tasks: 1
+is necessary! 
+Embulk will optimize the task according the core number which means that, it will re-distribute the task, which will cause 
+errors.
+
+If the multi_dir contains more than one directory. each directory will be treated as a task. the embulk will distribute those tasks to multi 
+thread. As each task will one consistently, the files in each directory will be uploading in order.
+For example the upload order maybe:
+example1.txt
+sample1.txt
+sample2.txt
+example2.txt
+sample3.txt
+
+To also upload the directory one by one, you need to configure the max_thread: 1
+then you will get
+example1.txt
+example2.txt
+sample1.txt
+sample2.txt
+sample3.txt
+
+
+
 For the order_by_modified_time option, its default value is 0, the files in each directory are uploaded in alphabetical order.
 If it equals 1, the files in each directory will be uploaded in the order of their last modified time. 
 if it equals neither 1 or 0, the files will be uploaded in descend order of their last modified time.