Document rpm layout format specification sources

[drahnr]

It would make sense to publicly state which implementation we follow, to make sure impl details are not arbitrary (besides "it works"). https://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/pkgformat.html https://xyrillian.de/thoughts/posts/argh-pm.html http://ftp.rpm.org/max-rpm/s1-rpm-file-format-rpm-file-format.html

[Richterrettich]

Yes, I'll add some documentation about it.

[dralley]

https://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/pkgformat.html

Note that "no rpm version from the last 20 years produces output that is actually compatible with LSB"

https://github.com/rpm-software-management/rpm/issues/1292#issuecomment-800302305