f2fs: add missing documentation for dir_level

This patch adds missing dir_level documentation.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/Documentation/ABI/testing/sysfs-fs-f2fs b/Documentation/ABI/testing/sysfs-fs-f2fs
index 4ac2c25..62dd725 100644
--- a/Documentation/ABI/testing/sysfs-fs-f2fs
+++ b/Documentation/ABI/testing/sysfs-fs-f2fs
@@ -56,6 +56,12 @@
 Description:
 		 Controls the number of trials to find a victim segment.
 
+What:		/sys/fs/f2fs/<disk>/dir_level
+Date:		March 2014
+Contact:	"Jaegeuk Kim" <jaegeuk.kim@samsung.com>
+Description:
+		 Controls the directory level for large directory.
+
 What:		/sys/fs/f2fs/<disk>/ram_thresh
 Date:		March 2014
 Contact:	"Jaegeuk Kim" <jaegeuk.kim@samsung.com>