Document the unconventional source layout

This package deviates from the Debian convention of native packages strictly using native package versions (without the Debian revision suffix) for practical reasons described in the Debian bug #684431. Since gbp does not support this workflow directly, it needs to be tricked into accepting it by having the same content on both upstream/* and debian/* branches just as if it were a non-native package.

Since native packages don’t have orig tarballs, but the non-native package version suggests there should be, the Debian tarball is used as one, but renamed accordingly, e.g. python3-defaults_3.9.2-2.tar.gz to python3-defaults_3.9.2.tar.gz.

When importing new versions, only import the new "upstream" versions (incrementing the version before dash) to upstream/* branches, while incremental updates (with changes in the Debian revision only) should go to debian/* only.

The package uses 1.0 format, so any new changes to the package with the same "upstream" version can be represented in the Debian diff, even if they touch files outside of debian/.

This merge request documents this situation directly in the source package.