Add a blurb about the attributes changes to the release notes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175075 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 67de945..2142162 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -59,6 +59,11 @@
 
 * We've improved the X86 and ARM cost model.
 
+* The Attributes classes have been completely rewritten and expanded. They now
+  support not only enumerated attributes and alignments, but "string"
+  attributes, which are useful for passing information to code generation. See
+  `How To Use Attributes <HowToUseAttributes.html>`_ for more details.
+
 * ... next change ...
 
 .. NOTE