Add v15.0.0 releasenote

This commit adds v15.0.0 releasenote. Because v15.0.0 was already
released.

Change-Id: Icc58ea6bd2e792a7fd38af6ef2f28d9cf5741a39
diff --git a/releasenotes/source/index.rst b/releasenotes/source/index.rst
index 242d133..cea76b4 100644
--- a/releasenotes/source/index.rst
+++ b/releasenotes/source/index.rst
@@ -6,6 +6,7 @@
     :maxdepth: 1
 
     unreleased
+    v15.0.0
     v14.0.0
     v13.0.0
     v12.0.0