To allow your plugin to be automatically installed by a user from the repository, you must develop it (or convert it) to use the autoinstall.php method of installation.
You must follow the Directory Layout for your plugin as outlined here Suggested_and_Required_Directory_Layout_and_Files.
As well, there are four files that must be included in the root directory of your plugin archive. These files are:
If your plugin includes these files, then it will be offered for auto install via the repository (and download and manual install if the user so wishes).
However, if any of these files are missing, a warning will be shown and the plugin will ONLY be offered for manual download. This can be fixed at any time, however, by simply editing your plugin in the repository, and uploading your fixed archive. Supported archives to the repository are .tar.gz, .tar, .tar.bz2, and .zip. Once you upload a plugin, it will be displayed when you go to the Repository Manager, and you will be able to edit it, upload patches for it, and upload upgrades for it, as well as remove it.