f2fs: add missing documentation for dir_level

This patch adds missing dir_level documentation.
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

Documentation/ABI/testing/sysfs-fs-f2fs

@@ -56,6 +56,12 @@ Contact:	"Jaegeuk Kim" <jaegeuk.kim@samsung.com>
 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>