fix: add missing dependencies to README.md

[tym2k1]

When building a demo inside of a docker container (especially ubuntu:latest) two additional dependencies must be downloaded through apt for make to work properly - file and cpio.

$ echo $PLATFORM
  qemu-aarch64-virt
$ echo $DEMO
  linux+zephyr
$ echo $CROSS_COMPILE
  /home/user/Desktop/bao/bao-demo/toolchain/arm-gnu-toolchain-13.2.Rel1-x86_64-aarch64-none-elf/bin/aarch64-none-elf-

You must install '/usr/bin/file' on your build machine
make[1]: *** [support/dependencies/dependencies.mk:27: dependencies] Error 1
make[1]: Leaving directory '/home/user/Desktop/bao/bao-demo/wrkdir/srcs/buildroot-aarch64-v6.1'
make: *** [/home/user/Desktop/bao/bao-demo/guests/linux/make.mk:28: /home/user/Desktop/bao/bao-demo/wrkdir/srcs/buildroot-aarch64-v6.1/output/images/Image-qemu-aarch64-virt] Error 2

make[2]: warning: -j1 forced in submake: resetting jobserver mode.
make[2]: Leaving directory '/home/user/Desktop/bao/bao-demo/wrkdir/srcs/buildroot-aarch64-v6.1'
You must install 'cpio' on your build machine
make[1]: *** [support/dependencies/dependencies.mk:27: dependencies] Error 1
make[1]: Leaving directory '/home/user/Desktop/bao/bao-demo/wrkdir/srcs/buildroot-aarch64-v6.1'
make: *** [/home/user/Desktop/bao/bao-demo/guests/linux/make.mk:28: /home/user/Desktop/bao/bao-demo/wrkdir/srcs/buildroot-aarch64-v6.1/output/images/Image-qemu-aarch64-virt] Error 2