Move source code into "Source" folder

Source/DataLayer/EfClasses/Supplement.cs

@@ -0,0 +1,26 @@
+using Dinah.Core;
+
+namespace DataLayer
+{
+    /// <summary>PDF/ZIP files only. Although book download info could be the same format, they're substantially different and subject to change</summary>
+    public class Supplement
+    {
+        internal int SupplementId { get; private set; }
+        internal int BookId { get; private set; }
+
+        public Book Book { get; private set; }
+        public string Url { get; private set; }
+
+        private Supplement() { }
+        public Supplement(Book book, string url)
+        {
+            ArgumentValidator.EnsureNotNull(book, nameof(book));
+            ArgumentValidator.EnsureNotNullOrWhiteSpace(url, nameof(url));
+
+            Book = book;
+            Url = url;
+        }
+
+		public override string ToString() => $"{Book} {Url.Substring(Url.Length - 4)}";
+	}
+}