Add URL of PCB files to README.md (#3182)

keyboards/bigswitch/README.md

@@ -3,6 +3,10 @@ Big Switch PCB by flehrad
 
 Designed by Don of the Board Podcast and sold as a kit by [keeb.io](https://keeb.io/collections/frontpage/products/big-switch-pcb?variant=7507922845726)
 
+### PCB Files
+
+https://github.com/flehrad/Big-Switch-PCB
+
 ### Technical Specifications
 
  * Uses a atmega32u4 pro micro or pin compatible MCU