document additional parameter

Author: hauner

docs/modules/ROOT/pages/mapping/parameter.adoc

@@ -100,3 +100,25 @@ the processor will use `java.time.ZonedDateTime` as java type for **all** parame
 **all** endpoints that have a "date" parameter.
 
 In the example both endpoints would use `java.time.ZonedDateTime` as java type for the "date" parameter.
+
+== additional parameter
+
+It is possible to add additional parameters that is not described in the OpenAPI. For example `HttpServletRequest` which may be needed to get something from the request that is not described in the api.
+
+While it is possible to add it at the global level, it is best used at the endpoint level.
+
+[source,yaml]
+----
+openapi-processor-mapping: v12
+
+options:
+  package-name: generated
+
+map:
+  paths:
+    /foo:
+      parameters:
+        - add: request => javax.servlet.http.HttpServletRequest
+----
+
+This will add a parameter `request` to the endpoint method.