Clarify that image files larger than 4GB must reside on exFAT-formatted SD card

README.md

@@ -34,6 +34,8 @@ Empty image files can be created using operating system tools:
 * Linux: `fallocate -l 1G HD1.img`
 * Mac OS X: `mkfile -n 1g HD1.img`
 
+If you need to use image files larger than 4GB, you _must_ use an exFAT-formatted SD card, as the FAT32 filesystem does not support files larger than 4,294,967,295 bytes (4GB-1 byte).
+
 ZuluSCSI firmware can also create image files itself.
 To do this, create a text file with filename such as `Create 1024M HD40.txt`.
 The special filename must start with "Create" and be followed by file size and the name of resulting image file.