Add note to README about gd and exif requirements

[asdfMaciej]

Hello,

I've noticed that images processed by this library aren't rotated correctly.

The original image with full metadata: https://kaszkowiak.org/IMG_0707.jpg

Resizing code:

$image = new ImageResize($path);
$image->crop(600, 600, true);
$image->save($path);

Result:

All related issues are closed, so I created a new one with a reproducible example :)

Thanks for creating and maintaining this library! Would love if this gets fixed, as currently it renders it unusable for me.

[asdfMaciej]

I've found the true cause. I didn't have the EXIF extension configured in PHP container:

docker-php-ext-install exif

Upon installing the extension, rotating the image works as expected!

I'm changing the issue title and leaving it open, as it would be really helpful if you mentioned it in README :)

Cheers!