Added <auto-generated> XML documentation tag to the generated file preamble.

Closes https://github.com/mono/CppSharp/issues/648.

src/Generator/Generators/CSharp/CSharpTextTemplate.cs

@@ -187,8 +187,10 @@ namespace CppSharp.Generators.CSharp
         {
             PushBlock(BlockKind.Header);
             WriteLine("//----------------------------------------------------------------------------");
+            WriteLine("// <auto-generated>");
             WriteLine("// This is autogenerated code by CppSharp.");
             WriteLine("// Do not edit this file or all your changes will be lost after re-generation.");
+            WriteLine("// </auto-generated>");
             WriteLine("//----------------------------------------------------------------------------");
             PopBlock();
         }