Add release notes for the overloadable attribute

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@307995 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ReleaseNotes.rst

@@ -82,7 +82,9 @@ Clang now supports the ...
 Attribute Changes in Clang
 --------------------------
 
--  ...
+-  The ``overloadable`` attribute now allows at most one function with a given
+   name to lack the ``overloadable`` attribute. This unmarked function will not
+   have its name mangled.
 
 Windows Support
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