doc: add features of LWG-3886

Author: X1aomu

README.md

@@ -17,6 +17,7 @@ Implemented LWG Issues:
 - [x] [LWG-3940](https://wg21.link/lwg3940) `std::expected<void, E>::value()` also needs `E` to be copy constructible
 - [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
 
 Enhancements: