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

llvm-svn: 341551
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 38895a0d80f3e1217300216fa7ad4a10b49b21db
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