Add screenshots in readme for Sylius plugin

[gabiudrescu]

I think it would help a lot people to see how this actually looks in a browser. Happy to contribute to this as a first issue.

[gplanchat]

As the interface still has a lot of fresh paint, I've got mixed feelings about adding it. What do you think @gabiudrescu ?

[gabiudrescu]

I am happy to add them and contribute with updates. My bet is that people resonate better with images than with tons of text. And feedback can come easier and better like this. Also, this can help see the evolution. Wouldn't you want in 5y from now to look down on the images and see how much better has this evolved?