document alreadyrouted apis. (#363)

Fixes #74

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

@@ -784,6 +784,16 @@ The Websocket Routing Filter runs if the url located in the `ServerWebExchangeUt
 
 Websockets may be load-balanced by prefixing the URI with `lb`, such as `lb:ws://serviceid`.
 
+=== Making An Exchange As Routed
+
+After the Gateway has routed a `ServerWebExchange` it will mark that exchange as "routed" by adding `gatewayAlreadyRouted`
+to the exchange attributes.  Once a request has been marked as routed, other routing filters will not route the request again,
+essentially skipping the filter.  There are convenience methods that you can use to mark an exchange as routed
+or check if an exchange has already been routed.
+
+* `ServerWebExchangeUtils.isAlreadyRouted` takes a `ServerWebExchange` object and checks if it has been "routed"
+* `ServerWebExchangeUtils.setAlreadyRouted` takes a `ServerWebExchange` object and marks it as "routed"
+
 == Configuration
 
 Configuration for Spring Cloud Gateway is driven by a collection of `RouteDefinitionLocator`s.