Added note about qemu-user-static package dependency

Author: pfeerick

simpleimage/README.md

@@ -3,6 +3,9 @@
 This script builds a bootable disk image usable to boot a Pine64 to Linux in
 aarch64 mode.
 
+## Dependencies
+`qemu-user-static` is needed for creating and configuring the rootfs image. Failure to have this package installed will most likely result in a `cp: cannot stat '/usr/bin/qemu-aarch64-static': No such file or directory` type error.
+
 ## Minimal base image
 
 The simpleimage format is only a minimal disk with partition table, bootloader