Add an Animated GIF

Author: da-x

README.md

@@ -10,7 +10,7 @@ A Tetris example written in Rust using Piston.
 * No automatic restart after 'game over'
 * No change in falling piece speed
 
-<img src="doc/tetris.png">
+<img src="doc/tetris.gif">
 
 1) Install Rust from [main site](https://www.rust-lang.org).
 2) Perform the following: