Updated _Footer (markdown)

_Footer.md

@@ -3,3 +3,5 @@
 [itch.io](https://raysan5.itch.io/raylib "raylib page at itch.io")
   \|  
 [GitHub](https://github.com/raysan5/raylib "raylib repository on GitHub")
+  \|  
+[Discord](https://discord.gg/VkzNHUE)