Updated the generated output to include an "auto-generated" header

Fixes #535

Author: sharwell

src/GitVersionTask/AssemblyInfoBuilder/AssemblyInfoBuilder.cs

@@ -10,7 +10,17 @@ public string GetAssemblyInfoText(VersionVariables vars, string assemblyName)
         var v = vars.ToList();
 
         var assemblyInfo = string.Format(
-@"using System;
+@"//------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated by a tool.
+//     GitVersion
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//------------------------------------------------------------------------------
+
+using System;
 using System.Reflection;
 
 [assembly: AssemblyVersion(""{0}"")]