add copyright to the end

lib/github_changelog_generator.rb

@@ -87,6 +87,7 @@ class ChangelogGenerator
       puts log
     end
 
+    log += "\n\n*This file was generated by [github_changelog_generator](https://github.com/skywinder/Github-Changelog-Generator)*"
     output_filename = "#{@options[:project]}_changelog.md"
     File.open(output_filename, 'w') { |file| file.write(log) }