From Plex

Development Documentation

Before you start developing plugins please take a look at our Plugins in progress page. If you plan on working on a plugin I'd suggest you edit the page so that other know that work is being done. We would hate to see people doing double work.

Information on Creating Plugins Site Config is available.

A list of utilities and browser add ons that can help you with the process of scraping a website.

More documentation to follow as our API matures. Meanwhile you could find informations on the forum

Finishing and releasing

Once your plugin is nearing completion you can post some information for it on our Unsupported plugins page. If you feel that your plugin is ready for the App Store check out App Store Submission Guidelines.