Added a short initrd guide
Added a short initrd guide

Signed-off-by: Jerome Gravel-Niquet <ef6a975f7348c9e334b1a556525712a47dfacc4b@gmail.com>

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