Add <auto-generated> (#24)

src/ProxyInterfaceSourceGenerator/FileGenerators/ProxyAttributeGenerator.cs

@@ -1,4 +1,4 @@
-namespace ProxyInterfaceSourceGenerator.FileGenerators
+namespace ProxyInterfaceSourceGenerator.FileGenerators
 {
     internal class ProxyAttributeGenerator : IFileGenerator
     {
@@ -6,7 +6,16 @@
 
         public FileData GenerateFile()
         {
-            return new FileData($"{ClassName}.cs", $@"using System;
+            return new FileData($"{ClassName}.g.cs", $@"//----------------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated by https://github.com/StefH/ProxyInterfaceSourceGenerator.
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//----------------------------------------------------------------------------------------
+
+using System;
 
 namespace ProxyInterfaceGenerator
 {{