Better explain how to enter DFU mode

Starting from v0.3, the PD Buddy Sink is sold without SW1 installed.  It
might be frustrating to see the README mention setting the switch only
to find that there isn't actually a switch to set.

Now the README mentions that there might not be a switch, and what to do
if there isn't.  Also, it's more specific about what position for the
switch makes the device enter DFU mode.

README.md

@@ -34,8 +34,9 @@ This will give you a complete copy of the repository, including the ChibiOS
 submodule.
 
 You will also need to install some program to flash the firmware.  The simplest
-option is [dfu-util][], as it requires no extra hardware.  If you prefer to use
-SWD, you could also use [stlink][] or [OpenOCD][].
+option is [dfu-util][], as it requires no extra hardware (though either the
+Boot switch must be installed or two pads must be bridged).  If you prefer to
+use SWD, you could also use [stlink][] or [OpenOCD][].
 
 [toolchain]: https://launchpad.net/gcc-arm-embedded
 [dfu-util]: http://dfu-util.sourceforge.net/
@@ -58,12 +59,16 @@ the following:
 
 ### dfu-util
 
-Set the boot mode switch on the PD Buddy Sink to DFU mode and plug it into your
-computer.  Flash the firmware with:
+Set the Boot switch (SW1) on the PD Buddy Sink to the position not marked on
+the silkscreen to set the device to DFU mode.  If your Sink doesn't have a Boot
+switch, you can simply bridge the two long, close-together pads on the
+footprint for SW1 with a blob of solder to achieve the same effect.  Once the
+Sink is set to DFU mode, plug it into your computer.  Flash the firmware with:
 
     $ dfu-util -a 0 -s 0x08000000:leave -D build/pd-buddy-firmware.bin
 
-Don't forget to set the switch back to normal mode after unplugging the device.
+Don't forget to set the switch back to normal mode (or remove the solder blob)
+after unplugging the device.
 
 ### stlink