Guidelines on how to extend whitelist in README

[brettimus]

Hey! Awesome work!

So, quick moment of praise: I'm trying to slow down videos in foreign language films on Netflix to be able to be able to hear native speech better. This. 👏🏼 Is. 👏🏼 So. 👏🏼 Great. 👏🏼 For. 👏🏼 That.

Seriously, I'm so happy you made this, and especially for Safari. My computer's battery and resources thank you.

Anyways, if you have quick advice on the process of how to extend this plugin to whitelist other sites, I'd open a PR with extra docs in the README.

Maybe it's just as simple as finding the class name of the video controller, and adding the correct url patterns in the manifest? But I wanted to check with you first.

Love,

B