A zip folder is not required, you can simply make a folder named script_mymod in your add-ons folder and it will work fine. However, a zip file is needed if you want to distribute the mod because that keeps it all in one piece for people to download. In the folder you need a description.txt with at least one character in it, and either a client.cs or a server.cs (or both if you want) depending on what kind of mod it is.