Close #761: Integrate coveralls.io to show test coverage
Created by: ollieparanoid
Whether we do this or not has been discussed here, and no one spoke out against it: https://github.com/postmarketOS/pmbootstrap/issues/761
Preview of the README.md: https://github.com/postmarketOS/pmbootstrap/blob/feature/coveralls/README.md
Submitting the coverage is working: https://coveralls.io/github/postmarketOS/pmbootstrap