Update README with license details

Added license information for curriculum and media assets.

Author: amy-b

README.md

@@ -1,2 +1,20 @@
 # art-plus-code
 Website for the 2025 Art + Code.
+
+
+## License
+
+All curriculum, documentation, lessons, and media assets in this repository are
+licensed under the **Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)** license unless otherwise noted.
+
+This means you are free to share, remix, and adapt the materials for any purpose,
+including commercial use, as long as you:
+
+1. Provide appropriate attribution to The Processing Foundation  
+2. Indicate any changes made  
+3. Distribute derivative works under the same CC BY-SA 4.0 license
+
+See the [LICENSE](./LICENSE) file for full details.
+
+**Note:** Software code in this repository (if any) remains under the MIT License,
+which is compatible with CC BY-SA 4.0 for documentation and media.