Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -12,6 +12,13 @@ Steps:
 1. Add a method to the `ScipPtr` struct, this should in most cases return a `Result<YOU_RETURN_TYPE, Retcode>` from calling the `scip_call!` macro. Use the unsafe bindings from the `ffi` module to call SCIP methods.
 2. The Model<T> struct has many [states](https://docs.rs/russcip/latest/russcip/model/index.html), choose the state that is safe to call your `ScipPtr` method in.
 
+
+## Wrapping a SCIP plugin
+
+1. SCIP plugins are essentially interfaces. So first step would be to check the plugin's (fundamental) callbacks from SCIP's documentation and write an interface that represents it. Here's the [one](https://www.scipopt.org/doc/html/CONS.php) for the constraint handler plugin for example. 
+2. SCIP saves PLUGIN_DATA for each plugin type. This is what _russcip_ uses to wrap the struct that implements the interface. You'd need to add an `include_{PLUGIN_NAME}` method on `Model<ProblemCreated>` that takes a box of this interface and defines the C callbacks. As a reference you can take a look at the pricer plugin [implementation](https://github.com/scipopt/russcip/blob/main/src/pricer.rs) and it's include [method](https://github.com/scipopt/russcip/blob/e13d20d3e8594d3262f312304f4f2400de48da39/src/scip.rs#L703). 
+
+
 ## Code Quality 
 
 - Add documentation for any new methods/struct.