Add release notes

Author: imdj

clang/docs/ReleaseNotes.rst

@@ -116,6 +116,9 @@ Resolutions to C++ Defect Reports
 - Bumped the ``__cpp_constexpr`` feature-test macro to ``202002L`` in C++20 mode as indicated in
   `P2493R0 <https://wg21.link/P2493R0>`_.
 
+- Implemented `CWG2517 Useless restriction on use of parameter in `
+  `constraint-expression <https://cplusplus.github.io/CWG/issues/2517.html>`_.
+
 C Language Changes
 ------------------