add gameplay preview

Author: spotzero

README.md

@@ -6,6 +6,7 @@ I wanted to learn how to use the Amethyst Game Engine and improve my rust, so I
 
 **Spacewar!** has the honour of also being one of the very first computer game ever written, so it seems like a logical place to start when learning to use a game engine.
 
+![Preview of gameplay](https://github.com/spotzero/spacewars/raw/master/preview.gif "Preview")
 
 ## Credits
 
@@ -35,4 +36,4 @@ cargo make release
 
 ## License
 
-This software is licensed under the GNU General Public License v3.0.  See LICENSE.txt for the full restrictions.
+This software is licensed under the GNU General Public License v3.0.  See LICENSE.txt for the full restrictions.