Add v13.0.0 release note page

Now that tempest 13.0.0 has been pushed [1] for the newton release lets
create a distinct releasenotes page for it.

[1] https://pypi.python.org/pypi/Tempest/13.0.0

Change-Id: I75e822f2ad354e1276e5a523b794b60942069b0c
2 files changed