exfatprogs: add label.exfat usage in README

add label.exfat usage in README.

Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
diff --git a/README.md b/README.md
index 20e2a5b..c41e40b 100644
--- a/README.md
+++ b/README.md
@@ -49,4 +49,13 @@
     1. check the consistency.
         fsck.exfat /dev/sda1
     2. repair and fix.(preparing)
+
+- label.exfat:
+    Get or set volume label from a given device that formatted by exfat filesystem.
+
+Usage example:
+    1. get volume label.
+        label.exfat -g /dev/sda1
+    2. set new volume label.
+        label.exfat -s "new label" /dev/sda1
 ```