The most direct way to contribute to the Wolfram Language Plugin is subscribing to a paid license because the revenue helps me to assign more time to its development. Beside this, there are two things where you can help to make the plugin better
If you had a hard time understanding a feature or you could not find something in the documentation, then please create an issue on GitHub and explain in detail what can be improved.
If you found a bug, please also report it as an issue on GitHub. Note that critical exceptions are caught automatically and you can report them directly from within IntelliJ IDEA.