Add: more way to print "Hello World" in C# lang

Csharp/examples/StaticUsingCSharp.cs

@@ -0,0 +1,8 @@
+// use 'static' keyword to do not specifying 'Console' class
+using static System.Console;
+
+// use WriteLine() method without specifying 'Console' class
+WriteLine("Hello World");
+
+// Include this line to keep the console from closing right after printing "Hello World!" (without specifying 'Console' class)
+ReadKey();

Csharp/examples/TopLevelStatementCSharp.cs

@@ -0,0 +1,9 @@
+Console.WriteLine("Hello, World!");
+/*System.Console.WriteLine("Hello World!");
+can be used if you don't set up
+'using System;' at top of file
+"At compile time, the compiler injects top-level statements into a generated Main method,
+which remains the program's entry point.*/
+
+// Include this line to keep the console from closing right after printing "Hello World!"
+Console.ReadKey();