Add a link to the new PerformanceTips docs from the 3.7 release notes

llvm-svn: 231349
diff --git a/llvm/docs/ReleaseNotes.rst b/llvm/docs/ReleaseNotes.rst
index a3b234e..c977ebb 100644
--- a/llvm/docs/ReleaseNotes.rst
+++ b/llvm/docs/ReleaseNotes.rst
@@ -44,6 +44,11 @@
 * The minimum required Visual Studio version for building LLVM is now 2013
   Update 4.
 
+* A new documentation page, :doc:`Frontend/PerformanceTips`, contains a 
+  collection of tips for frontend authors on how to generate IR which LLVM is 
+  able to effectively optimize.
+
+
 * ... next change ...
 
 .. NOTE