Update README.md

Author: malczuuu

README.md

@@ -18,6 +18,7 @@ with exception handling in Spring Boot.
 
 - ✅ Automatic mapping of exceptions to responses with `Problem` objects compliant with [RFC 7807][rfc7807].
 - ✅ Mapping of exceptions extending `ProblemException` to responses with `Problem` objects.
+- ✅ Mapping of exceptions annotated with `@ProblemMapping` to responses with `Problem` objects.
 - ✅ Fallback mapping of `Exception` to `Problem` objects representing `500 Internal Server Error`.
 - ✅ Simple configuration thanks to Spring Boot autoconfiguration.