Add comment to clarify GRUB bootable CD-ROM setup step in Makefile CI

Author: andrewmcwatters

.github/workflows/makefile.yml

@@ -82,6 +82,7 @@ jobs:
         install System.map ../iso/boot/System.map
         install .config ../iso/boot/config
 
+    # https://www.gnu.org/software/grub/manual/grub/html_node/Making-a-GRUB-bootable-CD_002dROM.html
     - name: Make a GRUB bootable CD-ROM
       run: |
         sudo apt-get install -y mtools xorriso