Add empty release notes for 3.4.

The idea is that docs/ReleaseNotes.rst is 3.3 and will be copied to the
branch by the release manager just before creating the release
candidates.

This ReleaseNotes_34.rst will then be moved over ReleaseNotes.rst after
the 3.3 release.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181349 91177308-0d34-0410-b5e6-96231b3b80d8