Updated reference channel entities

ref/Entities/IModifiable.cs

@@ -0,0 +1,11 @@
+using System;
+using System.Threading.Tasks;
+
+namespace Discord
+{
+    public interface IModifiable<TModel>
+    {
+        /// <summary> Modifies one or more of the properties of this object. </summary>
+        Task Modify(Action<TModel> func);
+    }
+}