Add a license

[thijsbrentjens]

Ideally, we should have a permissive open source license, for the re-use of the code / docs here

[pvgenuchten]

Suggestion:

• MIT for code
• CC-by for docs

[justb4]

Looks good, though I mostly use GNU GPL v3 and CC-by (docs). But is up to Geonovum. I'lll be happy with MIT then.

[pvgenuchten]

thijs decides, paul implements

[thijsbrentjens]

docs: CC-by 4.0 code: Thijs legt voor aan Geonovum, voorkeur GNU GPL v3

[justb4]

Done.