Sorry, plugin API documentation is not written yet. You can use example
plugins bundled with tt-rss and other finished plugins as a starting
point. Ask on the forums if you need help with anything specific.
Plugins may render new preference panes or embed themselves into several
existing one, store data using simple key -> value data or directly in
the database, modify how articles are rendered, alter feed data, and
many more things.
List of hooks you can use is available in classes/pluginhost.php.
Example plugins are available in the attic and