docs: info on using recipe to build kernel
Mention the ability to use devtool to automate
the steps of building a Firecracker tailored kernel
image.
## Creating a kernel Image
+### Manual compilation
+
Currently, Firecracker supports uncompressed ELF kernel images on x86_64 while on
aarch64 it supports PE formatted images.
@@ -49,6 +51,23 @@ can boot:
1. Upon a successful build, you can find the kernel image under `./vmlinux`
(for x86) or `./arch/arm64/boot/Image` (for aarch64).
+### Use the provided recipe
+
+The kernel images used in our CI to test Firecracker's features are obtained by
+using the recipe inside devtool:
+
+```bash
+./tools/devtool build_kernel -c ./resources/microvm-kernel-x86.config -n 8
+```
+
+or
+
+```bash
+./tools/devtool build_kernel -c ./resources/microvm-kernel-arm64.config -n 8
+```
+
+on an aarch64 machine.
+
## Creating a rootfs Image
A rootfs image is just a file system image, that hosts at least an init
