doc: add features of LWG-4031

Signed-off-by: Yuming He <ComixHe1895@outlook.com>

Author: ComixHe

README.md

@@ -22,6 +22,7 @@ Implemented LWG Issues:
 - [x] [LWG-4026](https://wg21.link/lwg4026) Assignment operators of `std::expected` should propagate triviality
 - [x] [LWG-3877](https://wg21.link/lwg3877) incorrect constraints on const-qualified monadic overloads for `std::expected`
 - [x] [LWG-3886](https://wg21.link/lwg3886) Monad mo' problems
+- [x] [LWG-4031](https://wg21.link/lwg4031) `bad_expected_access<void>` member functions should be noexcept
 
 Enhancements: