document sdusb

Reference: /n/atom/patch/applied2013/sdusbman
Date: Sat Oct 12 09:11:27 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/man/3/sdloop	Sat Oct 12 09:11:21 2013
+++ /sys/man/3/sdloop	Sat Oct 12 09:11:21 2013
@@ -1,6 +1,6 @@
 .TH SDLOOP 3
 .SH NAME
-sdloop, addloop \- loopback storage device interface
+sdloop, addloop, sdusb \- loopback storage device interface
 .SH SYNOPSIS
 .nf
 .B bind -a #S /dev
@@ -14,6 +14,8 @@
 .B addloop
 .I letter
 .I unit
+.br
+.B usbsd
 .SH DESCRIPTION
 .I Sdloop
 allows a file to be used as a sd device.  By specifying a number
@@ -54,6 +56,13 @@
 .EX
 addloop l /tmp/data
 .EE
+.PP
+The
+.B usbsd
+script allows USB disks to be
+used as sd devices, and automatically partitions them.
+Drive letters are assigned starting with `u'
+for compatability with the boot process.  
 .SH SOURCE
 .B /sys/src/9/port/sdloop.c
 .br