 Add instructions to getting-started documentation
 Add instructions to getting-started documentation

    Adds instructions for:
    - Renaming the firecracker binary after downloading
    - Making the firecracker binary executable

Signed-off-by: Rob Devereux <d0a8e0554f9e4ef234762f791cd53aee8ec62ac1@gmail.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker