Licence: CC BY-NC-SA 4.0 (GRBL firmware stays GPLv3)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -11,3 +11,8 @@ A 12" kinetic sand table that draws patterns in sand. A steel ball rolls under g
 - `sand-table-v2-assembly.step`: the v2 assembly, for viewing in any CAD tool.
 - `design-notes.md`: how it works, dimensions, parts to buy, hardware list and print settings.
 - `sand-table-v1.solidcraft`, `mechanism-mockups.solidcraft`: earlier layout and mechanism studies.
+
+## Licence
+[CC BY-NC-SA 4.0](https://creativecommons.org/licenses/by-nc-sa/4.0/): you may print, change and share this, including the software and the board, as long as you credit it, don't sell it, and share changes under the same licence. See [LICENSE](LICENSE).
+
+The one exception is the GRBL firmware file in `software/firmware/`. It's the unmodified [GRBL 1.1h](https://github.com/gnea/grbl) build and stays under the GPL v3 ([COPYING-GRBL](software/firmware/COPYING-GRBL)).