Add notice that body params are not merged into Conn.params whne using cast and validate plug (#589)

Author: hamir-suspect

lib/open_api_spex/plug/cast_and_validate.ex

@@ -1,6 +1,8 @@
 defmodule OpenApiSpex.Plug.CastAndValidate do
   @moduledoc """
   Module plug that will cast and validate the `Conn.params` and `Conn.body_params` according to the schemas defined for the operation.
+  Note that when using this plug, the body params are no longer merged into `Conn.params` and must be read from `Conn.body_params`
+  separately.
 
   The operation_id can be given at compile time as an argument to `init`: