The extension manager let you publish your extensions, but you still have to manage the source code by yourself. If you want several developers to work on the same extension, you need a code manager like Subversion. Even if you work alone on your extension, Subversion helps you to work with cleaner methods.
Your extension source code is ready for hosting on piwigo.org, the directory created by piwigo.org administrator is currently empty, you have to fill it with your extension source code.
At this point your need a Subversion client. For Windows users, the most common Subversion client is TortoiseSVN (needs Windows to be restarted after install). For Linux users, the “svn” command line tool do the job the perfect way.
First you have to checkout the empty directory that was created by piwigo.org administrators.
In the “plugins” (or “themes”, depending on the type of extension) directory of your Piwigo local installation, in your file browser, right click, “SVN Checkout”. then give the URL provided by email, like http://piwigo.org/svn/extensions/CommentEditor in the preceding example.
If you work in team, you will have to “SVN update” your extension as often as possible.
If you want to add a feature in your extension or fix a bug, modify the files (and/or add/remove files) and commit them. Don't commit files one by one, unless you exactly know what you're doing, you should SVN commit the whole extension directory.