Update mailing list subscription recommendations
Page Joining the Package Maintainers suggested
subscribing to the package-announce mailing list.
Due to its great volume,
this is not generally a good idea.
Replaced that suggestion with a suggestion
to adjust the Watch settings in the Package Sources.
Package-announce, as well as similar scm-commits list,
are mentioned as an alternative.
Finding a section where this information can be located
required some changes in document structure,
which as an additional improvement
also allows giving a bit more visibility
for the fact that a productive Package Maintainer
does not necessarily have to own a package.
Resolves #36