Added a note in the README about board specs

README.md

 This is the circuit board for the PD Buddy Sink, a device which aims to make
 USB Power Delivery accessible to any electronics hobbyist.
 
+The board is designed for 2 oz copper.  It's probably best to use a 1.0 or 0.8
+mm thick PCB to make sure it's possible to solder the USB connector's pins.
+
 The PCB is made with [KiCad](http://kicad-pcb.org/), so you'll need that if you
 want to make any changes.