added gif; modified README to include it

[Vincibean]

Problem

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.

Some READMEs to take inspiration from:

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

Solves Issue #25

Solution

• Added a new folder (screenshots) to store the GIF
• Added a new GIF with an example game; the GIF was designed to showcase a short game where most (if not all?) game functionalities are shown
• Modified the README to include the GIF

Notes

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.

GIF