update readme to include useRef and examples of custom pause/play buttons

[adrian-burkhart]

I've updated the Readme to replace the usage of this.rap.audioEl with an implementation that uses useRef instead. I've also added an example of how to use the ref to add custom play, pause, and stop buttons since how to do that seems to be a common question that people have.

Example (imagine music playing): https://github.com/justinmc/react-audio-player/assets/59536294/e994d233-73aa-4093-8241-aaf30b153fcd