Choose and add a software licence

[Monarda]

We need to properly license our software. That means choosing an appropriate licence and add a LICENCE.md file.

[Monarda]

The first question to answer is whether we want a permissive or copyleft license?

[Monarda]

@patrick-austin I've decided to use the MIT license. What's your opinion?

[patrick-austin]

@Monarda I prefer GPL3

[Monarda]

I don't think I can resolve this until #4 (improve test coverage) is resolved