BeagleV-Fire: Add cape Verilog customization tutorial.

First draft of how to modify the cape's gateware using the cape Verilog template as starting point. This is walking you though the steps to get the cape gateware to flash one of the user LEDs without software intervention.

This does not go into creating a Verilog test bench using Verilator. I think I will keep this as a separate document to keep this one simple.

Please check for glaring omissions of steps.

I went through the steps as I was writing this however, I am not sure that anybody with an account can fork the gateware repository.