Add release notes page for 18.0.0

Add a new page for 18.0.0 release notes.

Change-Id: I360103aa9c6fe9c0a7cbc3a2075b88bc279a55b3
diff --git a/releasenotes/source/index.rst b/releasenotes/source/index.rst
index df1de46..d968a44 100644
--- a/releasenotes/source/index.rst
+++ b/releasenotes/source/index.rst
@@ -6,6 +6,7 @@
     :maxdepth: 1
 
     unreleased
+    v18.0.0
     v17.0.0
     v16.1.0
     v16.0.0