Add release notes page for v10.0.0

This commit adds a seperate release notes page for tempest 10.0.0
to clearly mark what is inclued in that.

Change-Id: Ia3e4672777e6f2fbf5823c3869fa33715db2f2bf
diff --git a/releasenotes/source/index.rst b/releasenotes/source/index.rst
index 584540b..5bd9176 100644
--- a/releasenotes/source/index.rst
+++ b/releasenotes/source/index.rst
@@ -5,6 +5,7 @@
  .. toctree::
     :maxdepth: 1
 
+    v10.0.0
     unreleased
 
 Indices and tables