update CONTRIBUTING.md section about syn

Author: pvdrz

CONTRIBUTING.md

@@ -323,14 +323,12 @@ generated Rust code are implemented using the [`syn`](https://docs.rs/syn) crate
 
 ### Implementing new options using `syn`
 
-Here is a list of recommendations to be followed if a new option can be
-implemented using the `syn` crate:
+If a new option can be implemented using the `syn` crate it should be added to
+the `codegen::postprocessing` module by following these steps:
 
-- The `BindgenOptions::require_syn` method must be updated to reflect that this
-  new option requires parsing the generated Rust code with `syn`.
-
-- The implementation of the new option should be added at the end of
-  `Bindings::generate`, inside the `if options.require_syn() { ... }` block.
+- Introduce a new `struct` with no fields.
+- Implement the `PostProcessing` trait for the `struct`.
+- Add the `struct` to the `decl_postprocessing` macro invocation.
 
 ## Pull Requests and Code Reviews