On the right you will the list of the documents published:

• An introduction to plugin development, with an "hello world" type example.
• Manage Plugin Settings, explaining how to persist data specific to a plugin
• Create a custom settings page, with the instructions on how to build a custom settings page

If you are interested in how the plugin framework works you can have a look at the developer's wiki.

The wiki is not updated with the latest implementation details, but provides a pretty good overview of the general idea behind the Plugin Framework

More documentation will be published in a few weeks, but if you want to know when this site is updated you can subscribe to the blog of the developer that is responsible for the Subtext Plugin SDK: Simone Chiaretta a.k.a. CodeClimber.