add license to readme

README.md

@@ -18,4 +18,7 @@ It's also very handy to flash the bootloader on many Atmega328P IC's.
 
 - [Schematic](schematic.pdf)
 - [BOM](http://htmlpreview.github.io/?https://github.com/BenRoe/atmega328p-standalone-board/blob/master/bom/ibom.html)
-- [Gerber files](gerber/328p-standalone-rev1.0.zip) (ready to use)
+- [Gerber files](gerber/328p-standalone-rev1.0.zip) (ready to use)
+
+## License
+[![CC4](https://mirrors.creativecommons.org/presskit/buttons/88x31/svg/by-nc-sa.svg)](http://creativecommons.org/licenses/by-nc-sa/4.0/)