Add a coverage badge

Now that the test structure is (nearly) in place, we can test the coverage rate (which is quite bad at the moment) and add a badge on the README page.

I see 3 options:

• generate the badge and store it somewhere. I've tried to do this and store it on the git repo, the code is in coverage_badges branch of my fork. The problem is that I can't load the image trough gitlab (Content-type is always text for security issues), so the image needs to be copied elsewhere (where ?).
• use a service like coveralls/shields.io, but it relies on a external service (possibly closed source)
• use the badge generation from gitlab, but we haven't setup the CI yet 🤔

I'd rather follow the last option, @moul what do you think?