You can use the comment layer.
Alexander.
I wonder if there's preloaded symbols in the KiCad library too.
I know the standard way is to show the license on README.md and COPYRIGHT.md, putting the complete license on LICENSE.md or (NAME Of LICENSE).md for complex projects with GPL/LGPL/BSD for software and other like CERN Open Hardware License for the hardware schematics & PCB design.
It would be to have in kicad-library pretty symbols about "Made with KiCad" "X license" and such in different styles and sizes. Just to show the proud of using KiCad and being an Open Hardware project to the rest.
Please: No references to OSHWA and their OSHW tax :/