The plugins that are included in this package, are automatically added. That means you don't have to specify any <loader>-tag in the config to use these. You can en- or disable them by just using the <plugin>-tag.
TODO: List included plugins
Plugins has to inherit from DB_DataObject_FormBuilder_Frontend_Plugin
For the internal automatic loading of plugin-files to work, the file has to be named the same as the basename of the class. So, if the class containing your plugin is My_Frontend_Plugin_CoolPlugin it should be in a file named CoolPlugin.php (watch case!)
Where the file containing your plugin is, doesn't really matter. To use it, add this to the config:
The example above would requre that somewhere in the includepath was a file my/plugins/CoolPlugin.php containing a class named My_Frontend_Plugin_CoolPlugin