diff --git a/Documentation/md.txt b/Documentation/md.txt
index 7b3d471..b8d172b 100644
--- a/Documentation/md.txt
+++ b/Documentation/md.txt
@@ -200,6 +200,14 @@
      This can be written only while the array is being assembled, not
      after it is started.
 
+   new_dev
+     This file can be written but not read.  The value written should
+     be a block device number as major:minor.  e.g. 8:0
+     This will cause that device to be attached to the array, if it is
+     available.  It will then appear at md/dev-XXX (depending on the
+     name of the device) and further configuration is then possible.
+
+
 As component devices are added to an md array, they appear in the 'md'
 directory as new directories named
       dev-XXX