add note about amd64 prereqs for building

README.md

@@ -4,6 +4,18 @@
 
 Tool for building Butter Box disk images.
 
+## Host prerequisites
+
+The build host needs `vmdb2` and, for the amd64 image, the `syslinux-common`
+package — the amd64 recipe writes GPT protective-MBR boot code
+(`/usr/lib/syslinux/mbr/gptmbr.bin`) directly to the output image from the host
+filesystem, so this package must be present on the build machine (not just
+inside the target image):
+
+```sh
+sudo apt install syslinux-common
+```
+
 ## Usage
 
 To build a butter image, run: