diff --git a/HACKING.md b/HACKING.md
index 7f7c83f..ba1aac6 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -34,6 +34,8 @@ This will generate two firmware:
 - _bootloader_: a USB DFU bootloader
 - _application_: the main application
 
+*IMPORTANT*: to build the firmware for BusVoodoo hardware version 0, prepend `BUSVOODOO_HARDWARE_VERSION=0` before every `rake` command. This will make sure the few pin assignment differences are taken into account in the firmware.
+
 documentation
 -------------