suggest contributing in the "where to go next" page

Author: avivace

src/next.md

@@ -8,3 +8,4 @@ I'm actively working on fixing that, though, please be a little patient :)
 In the meantime, the best course of action is to peruse the [resources](resources.html) in the next section, and experiment by yourself.
 Well, given that, it may be a good idea to [ask around](help-feedback.html) for advice.
 A lot of the problems and questions you will be encountering have already been solved, so others can—and will!—help you getting started faster.
+If you enjoyed the tutorial, please consider [contributing](https://github.com/gbdev/gb-asm-tutorial).