Added a link to 'Making a game with Minecraft'

I have added a link to the 'making a game with minecraft' learning resource.

I didnt know whether to use a 'relative' link ie [link](../making-a-game-with-minecraft-pi/) or an absolute [link](http://www.raspberrypi.org/learning/making-a-game-with-minecraft-pi/) - I didnt know which was 'acceptable'?