Add stock example on Observer pattern

src/ObserverPattern/StockUpdateEvents/AaplObserver.cs

@@ -0,0 +1,29 @@
+using System;
+
+namespace ObserverPattern.StockUpdateEvents
+{
+    /// <summary>
+    /// Observer
+    /// </summary>
+    public class AaplObserver
+    {
+        double? oldValue;
+        public AaplObserver(StockSubject stockObservable)
+        {
+            stockObservable.StockUpdated += (obj, e) => PrintNewValue(e.Stock);
+        }
+
+        private void PrintNewValue(Stock stock)
+        {
+            if (stock.Name == "AAPL")
+            {
+                if (oldValue.HasValue)
+                    Console.WriteLine("Apple price updated from {0} to {1}", oldValue, stock.Value);
+                else
+                    Console.WriteLine("Apple has a new price, new value is: {0}", stock.Value);
+
+                oldValue = stock.Value;
+            }
+        }
+    }
+}