Add an open source license

[steffen]

No description provided.

[steffen]

I have been looking into this for the last couple of weeks and I think we should go with the MIT License in order to keep it simple for the start and to unblock allowing code contributions.

Here is a great overview of the MIT License with a comparison to a few others:
https://fossa.com/blog/open-source-licenses-101-mit-license/

According to the article it is the most popular open source license as of the writing (a year ago).
It is a simple, short, permissive, explicit and easy to understand license.
It is hard to foresee if we need or want to go with a more restrictive license later and to my understanding the MIT License would allow that.

A few resources I used to look up open source software licenses:

• https://choosealicense.com/
• https://fossa.com/blog/apply-license-open-source-software-project/
• https://en.wikibooks.org/wiki/FOSS_Licensing/How_to_Make_the_Source_Free_or_Open