Add "unreleased" release notes page

We need a release notes page to show notes for unreleased versions by
scanning the "current" branch. This ensures that notes files cannot be
merged if they break the releasenotes job.

Change-Id: Ic0614c37a67ebd28d1da0c0947c69c753833c870
Signed-off-by: Kyle Mestery <mestery@mestery.com>

releasenotes/source/index.rst

@@ -6,3 +6,4 @@
    :maxdepth: 1
 
    liberty
+   unreleased

releasenotes/source/unreleased.rst

@@ -0,0 +1,5 @@
+==============================
+ Current Series Release Notes
++==============================
+
+.. release-notes::