Add release note

Author: ericastor

clang/docs/ReleaseNotes.rst

@@ -237,6 +237,8 @@ Non-comprehensive list of changes in this release
 - The floating point comparison builtins (``__builtin_isgreater``,
   ``__builtin_isgreaterequal``, ``__builtin_isless``, etc.) and
   ``__builtin_signbit`` can now be used in constant expressions.
+- Plugins can now define custom attributes that apply to statements
+  as well as declarations.
 
 New Compiler Flags
 ------------------