Add documentation for "events" mixin

docs/extend/plugins/event.md

@@ -1,3 +1,36 @@
 ---
 title: Event Mixin
 ---
+
+## EventMixin
+
+The `EventMixin` class enables plugins to respond to certain triggered events.
+
+When a certain (server-side) event occurs, the background worker passes the event information to any plugins which inherit from the `EventMixin` base class.
+
+Implementing classes must provide a `process_event` function:
+
+```python
+class EventPlugin(EventMixin, IntegrationPluginBase):
+    """
+    A simple example plugin which responds to events on the InvenTree server.
+    
+    This example simply prints out the event information. 
+    A more complex plugin could respond to specific events however it wanted.
+    """    
+
+    PLUGIN_NAME = "EventPlugin"
+    PLUGIN_SLUG = "event"
+    PLUGIN_TITLE = "Triggered Events"
+
+    def process_event(self, event, *args, **kwargs):
+        print(f"Processing triggered event: '{event}'")
+```
+
+### Events
+
+Events are passed through using a string identifier, e.g. 'build.completed'
+
+The arguments (and keyword arguments) passed to the receiving function depend entirely on the type of event.
+
+Implementing a response to a particular event requires a working knowledge of the InvenTree code base, especially related to that event being received.