Add documentation

doc/development/api/plugins.rst

@@ -121,6 +121,7 @@ This will automatically make pretix discover this plugin as soon as it is instal
 through ``pip``. During development, you can just run ``python setup.py develop`` inside
 your plugin source directory to make it discoverable.
 
+.. _`Signals`:
 Signals
 -------
 
@@ -153,6 +154,28 @@ in the ``installed`` method:
 Note that ``installed`` will *not* be called if the plugin is indirectly activated for an event
 because the event is created with settings copied from another event.
 
+.. _`Registries`:
+Registries
+----------
+
+Many signals in pretix are used so that plugins can "register" a class, e.g. a payment provider or a
+ticket renderer.
+
+However, for some of them (types of :ref:`Log Entries <logging>`) we use a different method to keep track of them:
+In a ``Registry``, classes are collected at application startup, along with a unique key (in case
+of LogEntryType, the action_type) as well as which plugin registered them.
+
+To register a class, you can use one of several decorator provided by the Registry object:
+
+.. code-block:: python
+
+   @log_entry_types.new('my_pretix_plugin.some.action', _('Some action in My Pretix Plugin occured.'))
+   class MyPretixPluginLogEntryType(EventLogEntryType):
+       pass
+
+All files in which classes are registered need to be imported in the ``AppConfig.ready`` as explained
+in `Signals`_ above.
+
 Views
 -----