[libc++] Add a link to the Release notes from the main libc++ documentation

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@341551 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/index.rst b/docs/index.rst
index e4b3a87..1892cd4 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -34,6 +34,7 @@
 .. toctree::
    :maxdepth: 2
 
+   ReleaseNotes
    UsingLibcxx
    BuildingLibcxx
    TestingLibcxx