Added license.

Source/OpenTK/ContextHandle.cs

@@ -1,4 +1,12 @@
-using System;
+#region --- License ---
+/* Licensed under the MIT/X11 license.
+ * Copyright (c) 2006-2008 the OpenTK Team.
+ * This notice may not be removed from any source distribution.
+ * See license.txt for licensing detailed licensing details.
+ */
+#endregion
+
+using System;
 using System.Collections.Generic;
 using System.Text;
 
@@ -38,6 +46,9 @@ namespace OpenTK
             return Handle.GetHashCode();
         }
 
+        /// <summary>A read-only field that represents a handle that has been initialized to zero.</summary>
+        public static readonly ContextHandle Zero = new ContextHandle(IntPtr.Zero);
+
         /*
         public override bool IsInvalid
         {