tcl/interface/ftdi/swd-resistor-hack: clarify and add schematic diagram

Change-Id: I8600ee983de85e4225430ae508a50cd938122d89
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2357
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>

tcl/interface/ftdi/swd-resistor-hack.cfg

@@ -3,6 +3,23 @@
 # so depending on the drive capability of the target and adapter);
 # connect TDO directly to SWDIO.
 #
+# You also need to have reliable GND connection between the target and
+# adapter. Vref of the adapter should be supplied with a voltage equal
+# to the target's (preferrably connect it to Vcc). You can also
+# optionally connect nSRST. Leave everything else unconnected.
+#
+# FTDI                          Target
+# ----                          ------
+# 1  - Vref   ----------------- Vcc
+# 3  - nTRST  -
+# 4  - GND    ----------------- GND
+# 5  - TDI    ---/\470 Ohm/\--- SWDIO
+# 7  - TMS    -
+# 9  - TCK    ----------------- SWCLK
+# 11 - RTCK   -
+# 13 - TDO    ----------------- SWDIO
+# 15 - nSRST  - - - - - - - - - nRESET
+#
 
 transport select swd