Documentation: convert ReleaseNotes.html to reST.

Patch by Anthony Mykhailenko with small fixes by me.

llvm-svn: 169714
diff --git a/llvm/docs/userguides.rst b/llvm/docs/userguides.rst
index 81506a1..cfb6dbe 100644
--- a/llvm/docs/userguides.rst
+++ b/llvm/docs/userguides.rst
@@ -22,6 +22,7 @@
    Phabricator
    TestingGuide
    tutorial/index
+   ReleaseNotes
 
 * :ref:`getting_started`
     
@@ -65,7 +66,7 @@
 
    A list of common questions and problems and their solutions.
     
-* `Release notes for the current release <ReleaseNotes.html>`_
+* :doc:`Release notes for the current release <ReleaseNotes>`
 
    This describes new features, known bugs, and other limitations.