add note about accessing params in operation steps

gems/operation/2.0/api.md

@@ -27,6 +27,20 @@ Ruby allows a shorthand for this which is commonly used throughout Trailblazer.
 
 The absence of a method name here is per design: this object does only one thing, and hence *what it does is reflected in the class name*.
 
+Arguments passed to `call` will be available within steps (see below) under `options[:params]`:
+
+    # within Song::Create:
+    step :log_name!
+
+    def log_name!(options)
+      logger.write "creating song called #{options['params']['name']}"
+    end
+
+    # alternatively with Ruby's double splat operator:
+    def log_name!(options, params:, **)
+      logger.write "creating song called #{params[:name]}"
+    end
+
 Running an operation will always return its [→ result object](#result-object). It is up to you to interpret the content of it or push data onto the result object during the operation's cycle.
 
     result = Create.call( name: "Roxanne" )