Add badges for CI status and code coverage

[goerz]

For my convenience, as part of openjournals/joss-reviews#10384:

Could you add the badges that most Julia projects have at the top of the README that track the success of the main CI pipeline (the tests), and, more importantly, the code coverage on Codecov. Verified code coverage > 80% (ideally > 90%) is one of my primary metric to decide whether a package is sufficiently tested.

[darnstrom]

Thank you for the suggestion! This have been added in #94. I will add some extra test to cover some of the code that is currently not handled.