Update readme to mention GD support requirement

I had issues trying to use this library initially, because I didn't realize it was using GD manipulations under the hood, and by default my (and possibly most) php installations don't include GD support out of the box. 

I found the documentation and comments at http://php.net/manual/en/image.installation.php helpful in order to install `php5-gd` on my debian-based distro in order to use this library. 

Without GD support, errors occur such as: 

    Call to undefined function League\ColorExtractor\imagecreatefrompng() in [...]/vendor/league/color-extractor/src/League/ColorExtractor/Client.php on line 14

In order to help others, I submit this README.md update comment referencing the php.net documentation.