Add stock example on Observer pattern

src/ObserverPattern/StockUpdateEvents/Stock.cs

@@ -0,0 +1,10 @@
+namespace ObserverPattern.StockUpdateEvents
+{
+    public class Stock
+    {
+        public string Name { get; set; }
+        public int Quantity { get; set; }
+
+        public double Value { get; set; }
+    }
+}