Add demo project

CliFx.Demo/Models/Library.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+
+namespace CliFx.Demo.Models
+{
+    public partial class Library
+    {
+        public IReadOnlyList<Book> Books { get; }
+
+        public Library(IReadOnlyList<Book> books)
+        {
+            Books = books;
+        }
+    }
+
+    public partial class Library
+    {
+        public static Library Empty { get; } = new Library(Array.Empty<Book>());
+    }
+}