Add release notes page for version 20.0.0

Tempest 20.0.0 has been released [1].
This commit adds a new page for 20.0.0 release notes.

[1] https://review.openstack.org/#/c/645867/

Change-Id: Ic946b84e0003cc4cca85c1922816b8a0a3b8a3bf
diff --git a/releasenotes/source/index.rst b/releasenotes/source/index.rst
index 3be014f..e5d5bfe 100644
--- a/releasenotes/source/index.rst
+++ b/releasenotes/source/index.rst
@@ -6,6 +6,7 @@
    :maxdepth: 1
 
    unreleased
+   v20.0.0
    v19.0.0
    v18.0.0
    v17.2.0