Update README.md

Author: malczuuu

problem4j-spring-web/README.md

@@ -1,5 +1,15 @@
 # Built-in Spring Exception Mappings
 
+1. [Overview](#overview)
+2. [Returning response bodies from custom exceptions](#returning-response-bodies-from-custom-exceptions)
+    1. [Extending `ProblemException`](#extending-problemexception)
+    2. [Annotating `@ProblemMapping`](#annotating-problemmapping)
+3. [Validation](#validation)
+4. [Occurrences of `TypeMismatchException`](#occurrences-of-typemismatchexception)
+5. [Occurrences of `ErrorResponseException`](#occurrences-of-errorresponseexception)
+6. [General HTTP Stuff](#general-http-stuff)
+
+## Overview
 This module overrides Spring Web's default (often minimal or plain-text) responses for many framework exceptions and
 produces structured RFC 7807 `Problem` objects. [`ExceptionMappingConfiguration`][ExceptionMappingConfiguration]
 registers these mappings.