Add screenshot to README

[myroslambda]

It would be great to include more detail in this README.

In particular, something that demonstrates a game well is a screenshot or, if possible, a GIF.

So, let's try to get either of those and add them to the README :)

[chriz-zeller]

To get some inspiration you might also want to take a look at these READMEs:

• https://github.com/ivanperez-keera/haskanoid
• https://github.com/ivanperez-keera/SpaceInvaders

[Vincibean]

@myroslambda @chriz-zeller I just pushed #31 to solve this.
As stated in the PR, I tried to have the GIF showcase a short game where most (if not all?) game functionalities are shown; moreover, while it would have been possible to add a couple of screenshots as well, I chose not include them as I found them redundant: the GIF already shows everything a screenshot could show.
Don't hesitate to let me know if you'd rather have a different GIF, or would like to include some screenshots, or anything else.

[myroslambda]

@Vincibean I will be merging this one. Thank you!