ScriptAssetProcessor.cs

using UnityEngine;
using UnityEditor;
using System.IO;


public class ScriptAssetProcessor : UnityEditor.AssetModificationProcessor
{
  public static void OnWillCreateAsset(string path)
  {
    string filePath = GetAbsoluteFilePath(path.Replace(".meta", ""));

    if(!FileIsScript(filePath)) return;
    
    // Add a demo comment to the top of scripts
    string fileContent = System.IO.File.ReadAllText(filePath);
    fileContent = "// Demo!" + System.Environment.NewLine + fileContent;
    System.IO.File.WriteAllText(filePath, fileContent);

    AssetDatabase.Refresh(); // Force a script recompile 
  }

  static bool FileIsScript(string filePath)
  {
    var extension = System.IO.Path.GetExtension(filePath);
    return (extension == ".cs" || extension == ".js" || extension == ".boo");
  }

  static string GetAbsoluteFilePath(string path)
  {
    var index = Application.dataPath.LastIndexOf("Assets");
    return Application.dataPath.Substring(0, index) + path;
  }
}