add csharp version to the exercice

csharp/TheatricalPlayersRefactoringKata/Invoice.cs

@@ -0,0 +1,20 @@
+using System.Collections.Generic;
+
+namespace TheatricalPlayersRefactoringKata
+{
+    public class Invoice
+    {
+        private string _customer;
+        private List<Performance> _performances;
+
+        public string Customer { get => _customer; set => _customer = value; }
+        public List<Performance> Performances { get => _performances; set => _performances = value; }
+
+        public Invoice(string customer, List<Performance> performance)
+        {
+            this._customer = customer;
+            this._performances = performance;
+        }
+
+    }
+}