Add License information

[Thorbijoern]

Hi,
great project.
Everywhere on the website and in press coverage it is stated that the project is open source and open hardware, but i had to search a while to find the licence for the hardware: https://certification.oshwa.org/de000125.html
The link is only a small side note on the Website.
It would be great if the licence file could be added to the repository and the Licence could be mentioned in the Readme too.
The repository is most times the first place where technical people look for the license.
That way the licensing is more transparent and easy to find.
Thanks!