diff --git a/firmware/README b/firmware/README
index 0e47a391..fc398a6b 100644
--- a/firmware/README
+++ b/firmware/README
@@ -31,6 +31,10 @@ $ hackrf_spiflash -w hackrf_usb.bin
 If you have a Jawbreaker, add -DBOARD=JAWBREAKER to the cmake command.
 If you have a rad1o, use -DBOARD=RAD1O instead.
 
+If you have a HackRF One and want to build optional PortaPack display support,
+also specify -DUSER_INTERFACE=PORTAPACK. If PortaPack hardware is not detected
+by hardware probing at start-up, the HackRF firmware will run without UI
+support.
 
 It is possible to use a USB Device Firmware Upgrade (DFU) method to load
 firmware into RAM.  This is normally only required to recover a device that has