Update 3.7 Release Note mentionning the non-optionality of the DataLayout

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 232677
diff --git a/llvm/docs/ReleaseNotes.rst b/llvm/docs/ReleaseNotes.rst
index c977ebb..c0d2ea1 100644
--- a/llvm/docs/ReleaseNotes.rst
+++ b/llvm/docs/ReleaseNotes.rst
@@ -44,10 +44,14 @@
 * 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 
+* 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.
 
+* The DataLayout is no longer optional. All the IR level optimizations expects
+  it to be present and the API has been changed to use a reference instead of
+  a pointer to make it explicit. The Module owns the datalayout and it has to
+  match the one attached to the TargetMachine for generating code.
 
 * ... next change ...