Update ReleaseNotes

git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_34@209030 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ReleaseNotes.rst

@@ -27,6 +27,13 @@ Clang web page, this document applies to the *next* release, not the current
 one. To see the release notes for a specific release, please see the `releases
 page <http://llvm.org/releases/>`_.
 
+What's New in Clang 3.4.2?
+==========================
+
+* Build fix for gcc 4.9.
+
+* Added missing addrspacecast support.
+
 What's New in Clang 3.4.1?
 ==========================