Added line to remind player to add the player file into the lib file if they are not writing it there already.

Author: Relu

src/intro/hello-world.md

@@ -264,7 +264,9 @@ Rust does not natively support inheritance, but the gdext API emulates it to a c
 
 ### Class declaration
 
-In this example, we declare a class called `Player`, which inherits [`Sprite2D`][api-class-sprite2d] (a node type):
+In this example, we declare a class called `Player`, which inherits Sprite2D (a node type).
+This can be either defined in `lib.rs` or in a separate file `player.rs`.
+In case you go for the latter, don't forget to declare mod player; in your lib.rs file.
 
 ```rust
 use godot::prelude::*;