Add note pointing to unit tests.

docs/src/main/asciidoc/spring-cloud-gateway.adoc

@@ -300,6 +300,8 @@ RemoteAddressResolver resolver = XForwardedRemoteAddressResolver
 
 Route filters allow the modification of the incoming HTTP request or outgoing HTTP response in some manner. Route filters are scoped to a particular route. Spring Cloud Gateway includes many built-in GatewayFilter Factories.
 
+NOTE For more detailed examples on how to use any of the following filters, take a look at the https://github.com/spring-cloud/spring-cloud-gateway/tree/master/spring-cloud-gateway-core/src/test/java/org/springframework/cloud/gateway/filter/factory[unit tests].
+
 === AddRequestHeader GatewayFilter Factory
 The AddRequestHeader GatewayFilter Factory takes a name and value parameter.