Added docs for MatchBehaviour.RejectOnMatch

Stubbing-and-Request-Matching.md

@@ -158,6 +158,24 @@ server
 Url like /somebody and /someting
 ```
 
+### Reversing the match behaviour with `MatchBehaviour.RejectOnMatch`
+
+The default behaviour for Matchers is MatchBehaviour.AcceptOnMatch so that when the matcher processes a request that corresponds with the matcher, the stubbed response is returned. In some scenarios you might want to reverse this behaviour so that the stubbed response is returned with the absence of a match.
+
+e.g. You want to return `401 Unauthorised` if the caller does not provide a header containing the API Key:
+
+```csharp
+server
+   .Given(Request.Create()
+             .WithPath("/needs-a-key")
+             .UsingGet()
+             .WithHeader("api-key", "*", MatchBehaviour.RejectOnMatch)
+             .UsingAnyMethod())
+   .RespondWith(Response.Create()
+             .WithStatusCode(HttpStatusCode.Unauthorized)
+             .WithBody(@"{ ""result"": ""api-key missing""}"));
+```
+
 ## Response Templating
 Response headers and bodies can optionally be rendered using [Handlebars.Net](https://github.com/rexm/Handlebars.Net) templates.
 This enables attributes of the request to be used in generating the response e.g. to pass the value of a request ID header as a response header or render an identifier from part of the URL in the response body. To use this functionality, add `.WithTransformer()` to the response builder.