mk: add some details on how to build with touch sensing

gateware/Makefile

@@ -11,7 +11,7 @@ ifeq ($(BOARD),)
 	@echo "Valid HW_REV values are: $(ALL_HW_REV)".
 	@echo "Valid BOARD values are: $(ALL_BOARDS)".
 	@echo "Valid CORE values are: $(ALL_CORES)".
-	@echo "Valid TOUCH values are: $(ALL_TOUCH) (default disabled, valid for R3.3+ only)".
+	@echo "Valid TOUCH values are: $(ALL_TOUCH) (default disabled, valid for R3.3+ only, required for any examples that use touch)".
 	@echo "For example:"
 	@echo "  $$ make clean"
 	@echo "  $$ # Build bitstream with specific core and program it"

gateware/cores/touch_cv.sv

@@ -2,6 +2,11 @@
 // Touch-to-CV. Touches on input jacks 1-4 are
 // translated into CV outputs on outputs 1-4.
 //
+// When building with touch sensing, be sure to build with
+// TOUCH=TOUCH_SENSE_ENABLED, for example:
+//
+// $ make CORE=touch_cv HW_REV=HW_R33 TOUCH=TOUCH_SENSE_ENABLED
+//
 
 `default_nettype none