doc: Add a minimal rd.live.overlay documentation

Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>

dracut.cmdline.7.asc

@@ -838,6 +838,18 @@ can be found.  By default, this is __LiveOS__.
 Copy the complete image to RAM and use this for booting. This is useful
 when the image resides on i.e. a DVD which needs to be ejected later on.
 
+**rd.live.overlay=**_<devspec>_:_(<pathspec>|auto)__
+Allow the usage of a permanent overlay.
+_<devspec>_ specifies the path to a device with a mountable filesystem.
+_<pathspec>_ is the path to a file within that filesystem, which shall be used to
+persist the changes made to the device specified by **root=live:__<url>__** option.
++
+[listing]
+.Example
+--
+rd.live.overlay=/dev/sdb1:persistent-overlay.img
+--
+
 **rd.live.overlay.thin=**1::
 Enables the usage of thin snapshots instead of classic dm snapshots.
 The advantage of thin snapshots is, that they support discards, and will free