build: add make release target

It is useful to have as much as possible scripted, to be less error-prone
and to have a uniform and simple workflow. Therefore, make the tagging,
tarball and mail message in a uniform way.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
docs: add initial workflow document

Trying to document the workflow that comes along with using this Git
repository. Further ideas, additions to this document are welcome to
create a unified frame for all maintainers, developers and (advanced)
users.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>