diff --git a/README.md b/README.md
index 1cc8fc8..fd8da9e 100644
--- a/README.md
+++ b/README.md
@@ -86,6 +86,19 @@ type Options struct {
 	// If given, copy.Copy refers to this fs.FS instead of the OS filesystem.
 	// e.g., You can use embed.FS to copy files from embedded filesystem.
 	FS fs.FS
+
+	// NumberOfWorkers represents the number of workers used for
+	// concurrent copying contents of directories.
+	// If 0 or 1, it does not use goroutine for copying directories.
+	// Please refer to https://pkg.go.dev/golang.org/x/sync/semaphore for more details.
+	NumberOfWorkers int64
+
+	// PreferConcurrent is a function to determine whether or not
+	// to use goroutine for copying contents of directories.
+	// If PreferConcurrent is nil, which is default, it does concurrent
+	// copying for all directories.
+	// If NumberOfWorkers is 0 or 1, this function will be ignored.
+	PreferConcurrent func(srcdir, destdir string) (bool, error)
 }
 ```
 
@@ -105,4 +118,4 @@ err := Copy("your/directory", "your/directory.copy", opt)
 
 
 ## License
-[![FOSSA Status](https://app.fossa.com/api/projects/git%2Bgithub.com%2Fotiai10%2Fcopy.svg?type=large)](https://app.fossa.com/projects/git%2Bgithub.com%2Fotiai10%2Fcopy?ref=badge_large)
\ No newline at end of file
+[![FOSSA Status](https://app.fossa.com/api/projects/git%2Bgithub.com%2Fotiai10%2Fcopy.svg?type=large)](https://app.fossa.com/projects/git%2Bgithub.com%2Fotiai10%2Fcopy?ref=badge_large)