Instructions do not mention graphicsmagick

[davidair]

When trying to run the plugin out-of-the-box on a clean machine, I get the following error:

ImageMagick/GraphicsMagick is not installed

This can be addressed by something like brew install graphicsmagick

Can the instructions be updated to include this as a prerequisite?

[KrauseFx]

Great idea, can you submit a PR to the main README using https://github.com/KrauseFx/fastlane-plugin-appicon/edit/master/README.md