[Suggestion] Add usage instructions for first time jammers and beginner devs

[Maaack]

This repo can expect use by people with very little to no experience developing in Godot. It'd be helpful to have step-by-step instructions for how one would use the rebindable_action.gd in their own project.

[Maaack]

I updated the README a bit in #17 , but more detailed instructions (with screenshots) are welcome.