Fix instructions for new maintainers, incorporating the actual text

Fix instructions for new maintainers, incorporating the actual text
sent to prospective new maintainers.  Improve this text a bit for
readability, coverage, and organization.  Significant changes were
patched back to the new-maintainers group, if they care to use
them. (closes Bug#26948)

Add an introductory "Scope" section which helps delineate what should
and should not be included in this Reference.

Add discussion of the "experimental" distribution, culled from an
email from Guy Maor on debian-devel.

Point to doc-debian's mailing list instructions where relevant.

Made references to online documentation into URLs where possible.

Little corrections here and there.

git-svn-id: svn://anonscm.debian.org/ddp/manuals/trunk/developers-reference@655 313b444b-1b9f-4f58-a734-7bb04f332e8d