unix: Add comment about needed dependencies for MICROPY_FORCE_32BIT.

unix/Makefile

@@ -44,6 +44,8 @@ endif
 LDFLAGS = $(LDFLAGS_MOD) $(LDFLAGS_ARCH) -lm $(LDFLAGS_EXTRA)
 
 ifeq ($(MICROPY_FORCE_32BIT),1)
+# Note: you may need to install i386 versions of dependency packages,
+# starting with linux-libc-dev:i386
 CFLAGS += -m32
 LDFLAGS += -m32
 ifeq ($(MICROPY_PY_FFI),1)