Add note about -mfloat-abi=hard versus -mfloat-abi=soft

HACKING

@@ -44,6 +44,16 @@ over USB:
 The device should reset and re-enumerate as a CDC-ACM device implementing
 the GDB protocol.
 
+Errors when compiling libopencm3
+-------------------------------
+If while compiling libopencm3 you get an error like
+arm-none-eabi/bin/ld: error: cdcacm.elf uses VFP register arguments, \
+ arm-none-eabi/lib/thumb/v7m/libc.a(lib_a-memcpy-stub.o) does not
+your toolchain and libopencm3 disagree on the calling convention for floation
+point functions on the F4. Change in
+lib/stm32/f4/Makefile and examples/stm32/f4/Makefile.include all apperance of
+-mfloat-abi=hard to -mfloat-abi=soft
+This doesn't matter for blackmagic, as it doesn't use floating point.
 
 Compiling as a Linux application using FT2232 hardware
 ------------------------------------------------------