Add c# to rust example

csharp-to-rust/src/double.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Runtime.InteropServices;
+
+namespace DoubleClass {
+
+    class Double {
+        [DllImport ("libdouble_input")]
+        public static extern int double_input (int input);
+
+
+        public static void Main (string[] args) {
+            Console.WriteLine (double_input (2));
+        }
+    }
+}

csharp-to-rust/src/lib.rs

@@ -0,0 +1,4 @@
+#[no_mangle]
+pub extern fn double_input(input: i32) -> i32 {
+    input * 2
+}