Nasm is required when building hello world example

[Dawid Pogorzelski]

Hey, not sure if intentional but nasm is also needed for the hello world example:

 error: Failed to execute nasm: No such file or directory (os error 2)
     --> src/arch/x86_64/kernel/apic.rs:767:22
      |
  767 |     let smp_boot_code = include_nasm_bin!("boot.asm");
      |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      |
      = note: this error originates in the macro `include_nasm_bin` (in Nightly builds, run with -Z macro-backtrace for more info)

[Martin Kröning]

Thanks for the issue!

I have opened #301 and hermit-os/kernel#456 to clarify the requirements in the README. :)