[docs] Add php-vips-ext to README.md requirements section #762

Issue description

I use imagine with modern and fast graphic engine libvips via php-vips-ext and it work perfect. I just want for other people can see in main page about this possibility because VIPS is very popular.

What version of Imagine are you using?

imagine 1.2.4, libvips/php-vips-ext as low-level api and rokka-io/imagine-vips for high-level api

What's the PHP version you are using?

7.4

What's the imaging library you are using [gd/imagick/gmagick/any]?

libvips

What's the imaging library configuration

vary

P.S. Thanks to imagine authors and contributors for a great job!