Add stock example on Observer pattern

src/ObserverPattern/Unsubscriber.cs

@@ -0,0 +1,24 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace ObserverPattern
+{
+    public class Unsubscriber<T> : IDisposable
+    {
+        private List<IObserver<T>> _observers;
+        private IObserver<T> _observer;
+
+        public Unsubscriber(List<IObserver<T>> observers, IObserver<T> observer)
+        {
+            _observers = observers;
+            _observer = observer;
+        }
+
+        public void Dispose()
+        {
+            if (!(_observer == null)) _observers.Remove(_observer);
+        }
+    }
+}