Document Supported Image Formats per Platform

[udarrr]

Hello
Regarding part of code in your project like

const format = options.format || 'jpg' 

i can get image format whatever i want. I was able to check it and obtain *.bmp. It's great! But in your description was mentioned just 'png' | 'jpg'

[bencevans]

Sweet spot! Though, this is platform-dependent. Including a matrix in the README with known formats per platform might be handy.