Add release notes page for version 26.0.0

Change-Id: I0973b24dd006b8a96ddd2c11302e9b79ca7c4876
diff --git a/releasenotes/source/index.rst b/releasenotes/source/index.rst
index d8702f9..21f414e 100644
--- a/releasenotes/source/index.rst
+++ b/releasenotes/source/index.rst
@@ -6,6 +6,7 @@
    :maxdepth: 1
 
    unreleased
+   v26.0.0
    v24.0.0
    v23.0.0
    v22.1.0