GH-864 Add support for carying HTTP request parameters in Message headers

Resolves #864

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

@@ -650,6 +650,13 @@ When POSTing text the response format might be different with Spring Boot 2.0 an
 
 See <<Testing Functional Applications>> to see the details and example on how to test such application.
 
+==== HTTP Request Parameters
+As you have noticed from the previous table, you can pass an argument to a function as path variable (i.e., `/{function}/{item}`).
+For example, `http://localhost:8080/uppercase/foo` will result in calling `uppercase` function with its input parameter being `foo`.
+
+While this is the recommended approach and the one that fits most use cases cases, there are times when you have to deal with HTTP request parameters. 
+The framework will treat HTTP request parameters similar to the HTTP headers by storing them in `Message` headers under the header key `http_request_param` 
+with its value being a `Map` of request parameters, so in order to access them your function input signature should accept `Message` type (e.g., `Function<Message<String>, String>`). For convenience we provide `HeaderUtils.HTTP_REQUEST_PARAM` constant.
 
 === Function Mapping rules