Add comment to clarify initramfs image creation step in Makefile CI

Author: andrewmcwatters

.github/workflows/makefile.yml

@@ -57,6 +57,7 @@ jobs:
       run: |
         mkdir -p iso/boot
 
+    # https://docs.kernel.org/filesystems/ramfs-rootfs-initramfs.html
     - name: Create initramfs image
       run: |
         mkdir initramfs