Added instructions for Unity Unit Tests

[mrserverless]

Unit Tests calling ZeroFormatter will always return the error:

System.InvalidOperationException : Type is not supported, please register Player

After some hair scratching and re-reading your docs, I realised it's because the auto formatter registration for RuntimeInitializeOnLoad doesn't apply to unit tests. So by manually adding:

ZeroFormatterInitializer.Register(); 

In test SetUp works fine. I thus updated the README. Feel free to re-word it.

[neuecc]

thank you, yes, it is nice info.