add csharp version to the exercice

csharp/TheatricalPlayersRefactoringKata/Performance.cs

@@ -0,0 +1,18 @@
+namespace TheatricalPlayersRefactoringKata
+{
+    public class Performance
+    {
+        private string _playID;
+        private int _audience;
+
+        public string PlayID { get => _playID; set => _playID = value; }
+        public int Audience { get => _audience; set => _audience = value; }
+
+        public Performance(string playID, int audience)
+        {
+            this._playID = playID;
+            this._audience = audience;
+        }
+
+    }
+}