feat: Added attribute to easily register additional lua methods

Previously this could only be done by implementing a trait, like
`AddAdditionalMethods`, that that has an add_methods function where you
can put your custom methods. With this new attribute you can pass in a
register function directly!

automation_macro/src/lib.rs

@@ -32,26 +32,40 @@ pub fn lua_serialize(input: proc_macro::TokenStream) -> proc_macro::TokenStream
 
 /// Derive macro generating an impl for the trait `::mlua::UserData`
 ///
+/// # Device traits
 /// The `device(traits)` attribute can be used to tell the macro what traits are implemented so that
 /// the appropriate methods can automatically be registered.
 /// If the struct does not have any type parameters the syntax is very simple:
-/// ```
+/// ```rust
 /// #[device(traits(TraitA, TraitB))]
 /// ```
 ///
 /// If the type does have type parameters you will have to manually specify all variations that
 /// have the trait available:
-/// ```
+/// ```rust
 /// #[device(traits(TraitA, TraitB for <StateA>, <StateB>))]
 /// ```
 /// If multiple of these attributes are specified they will all combined appropriately.
 ///
 ///
-/// # NOTE
+/// ## NOTE
 /// If your type _has_ type parameters any instance of the traits attribute that does not specify
 /// any type parameters will have the traits applied to _all_ other type parameter variations
 /// listed in the other trait attributes. This behavior only applies if there is at least one
 /// instance with type parameters specified.
+///
+/// # Additional methods
+/// Additional methods can be added by using the `device(add_methods)` attribute. This attribute
+/// takes the path to a function with the following signature that can register the additional methods:
+///
+/// ```rust
+/// # struct D;
+/// fn top_secret<M: mlua::UserDataMethods<D>>(methods: &mut M) {}
+/// ```
+/// It can then be registered with:
+/// ```rust
+/// #[device(add_methods(top_secret))]
+/// ```
 #[proc_macro_derive(Device, attributes(device))]
 pub fn device(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
     let ast = parse_macro_input!(input as DeriveInput);