Added the initial matrix contracts file with a sample pre- and postcondion

@bsoetaer 
Ok I messed up how I read the assertion. I see it actually checks when the value is false as well. Guess I am not experienced enough in ruby yet and I am a little rusty on coding in general. I am still unsure where x, i, and j come from in the postcondition. At first glance I thought they were coming from result which is why I asked what result was. It looks fine to me now except I am still not sure where the matrix indices and values come from.