Adds markdown syntax highlight on first code block in readme

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README.md

@@ -43,7 +43,7 @@ Great introduction to Rx article: [The introduction to Reactive Programming you'
 
 The following code implements the double click detection example from the article in UniRx:
 
-```
+```csharp
 var clickStream = Observable.EveryUpdate()
     .Where(_ => Input.GetMouseButtonDown(0));