Replace puts with write in README.md for dir recursion

[Meatballs1]

The instructions for zipping a directory recursively uses puts:

  def put_into_archive(disk_file_path, io, zip_file_path)
    io.get_output_stream(zip_file_path) do |f|
      f.puts(File.open(disk_file_path, 'rb').read)
    end
  end

This appends a newline character 0x0A at the end of each file, which has just caused me a number of headaches :)

[coveralls]

Coverage decreased (-0.04%) to 93.972% when pulling c0dccb1 on Meatballs1:fix_readme_dir_recursion into 7aa3666 on rubyzip:master.