README: show how to define helper functions in a separate `module`

Developers might prefer to define helpers in a separate `module`, particularly
for modular apps, so let's provide an example in the docs.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>

README.rdoc

@@ -679,6 +679,22 @@ route handlers and templates:
     bar(params[:name])
   end
 
+Helper methods can be separately defined in a module:
+
+  module FooUtils
+    def bar(name)
+      "#{name}bar"
+    end
+  end
+
+  module MooUtils
+    # ...
+  end
+
+  helpers FooUtils, MooUtils
+
+The effect is same as including a module.
+
 === Using Sessions
 
 A session is used to keep state during requests. If activated, you have one