Add license

[mekline]

@eduardklap suggests adding a license, which we should do! Just opening an issue for now so we can decide which one to be used across the Psych-DS project.

[eduardklap]

I would suggest CC-BY as a good license for the Psych-DS project in general. For the software to be developed such as the Psych-DS validator, a specific software license such as the MIT license