Moving out of prototyping phase

[vil1]

Once all other issues labelled with needed-for-v1 are done, a few things need to be done in order to move out the prototyping phase:

• [ ] add file headers and other "administrative stuff" (most of this should be done in sbt-org-policies)
• [ ] add a proper CONTRIBUTING.md
• [ ] merge prototyping into master
• [ ] reconfigure the repository to make master the default branch again (CI, protected branches, etc)