Some small improvements for clarity.

* PostModel -> PostDto
* 404 test case
* README

service/PostDto.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace service;
+
+[Serializable]
+public class PostDto
+{
+    public int Id { get; set; }
+    public int UserId { get; set; }
+    public string Title { get; set; }
+    public string Body { get; set; }
+}