Release notes: mention support for Unicode and UCNs in identifiers.

I'm using the name "Extended Identifiers" for the feature because that's
what GCC calls them. According to the standard, the new feature is
"universal character names are now allowed in identifiers", but the more
interesting "feature" is that identifiers can now contain Unicode characters,
however they are written.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174798 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 3c09a58..5435ce5 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -56,6 +56,15 @@
 
 -  ...
 
+Extended Identifiers: Unicode Support and Universal Character Names
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Clang 3.3 includes support for *extended identifiers* in C99 and C++.
+This feature allows identifiers to contain certain Unicode characters, as
+specified by the active language standard; these characters can be written
+directly in the source file using the UTF-8 encoding, or referred to using
+*universal character names* (``\u00E0``, ``\U000000E0``).
+
 New Compiler Flags
 ------------------