v0.4.0

This release adds mostly automated tests so breakage is much easier to detect!

Important commits:
5b08a64 Add manifest to build response
4e8bc46 add GitHub actions
5c743d2 Create LICENSE
1737414 Add initial documentation via mkdocs
9da1e94 Implement branch system for versions
595a412 fixup: empty package list and diff_packages name
afcfb4b build: implement diff_packages