ReleaseNotes: Add section for R600 backend

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174764 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 78ea24e..274537c 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -87,6 +87,14 @@
 - Vectorization of function calls
 - Partial unrolling during vectorization
 
+R600 Backend
+------------
+
+The R600 backend was added in this release, it supports AMD GPUs
+(HD2XXX - HD7XXX).  This backend is used in AMD's Open Source
+graphics / compute drivers which are developed as part of the `Mesa3D
+<http://www.mesa3d.org>`_ project.
+
 
 
 Additional Information