Package-build infrastructure needs documentation

We need documentation to describe the package-build infrastructure such as http://arm01.puri.sm and https://source.puri.sm/Librem5/deb-build-jobs/. Readers should be able to understand how the build systems fit together and how to test changes to https://source.puri.sm/Librem5/deb-build-jobs/blob/master/jobs.yml prior to submission as merge requests.