Create README.md

Author: soheilkd

README.md

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+# SingleInstanceCore
+Single instance application - For WPF on .NET Core
+# Usage
+The App class (on App.xaml.cs) should inherit ISingleInstance and implement OnInstanceInvoked method:
+```csharp
+public partial class App : Application, ISingleInstance
+{
+	public void OnInstanceInvoked(string[] args)
+	{
+			//What to do with the args another instance has sent
+	}
+	...
+}
+```
+The initialization of the instance should be done on application startup or main window's startup
+Cleanup method should be called on the exit point of the application. 
+```csharp
+
+	private void Application_Startup(object sender, StartupEventArgs e)
+	{
+		bool isFirstInstance = SingleInstance<App>.InitializeAsFirstInstance("soheilkd_EPlayerIPC"))
+		if (!isFirstInstance)
+		{
+			//If it's not the first instance, arguments are automatically passed to the first instance
+			//OnInstanceInvoked will be raised on the first instance
+			//You may shut down the current instance
+			Current.Shutdown();
+		}
+	}
+		
+	private void Application_Exit(object sender, ExitEventArgs e)
+	{
+		//Do not forget to cleanup
+		SingleInstance<App>.Cleanup();
+	}
+```