Update README.md to document creating custom EventListeners

Author: strohganoff

README.md

@@ -284,6 +284,69 @@ And a command like the following is called by the Stream Deck software:
 streamdeck -port 28196 -pluginUUID 63831042F4048F072B096732E0385245 -registerEvent registerPlugin -info '{"application": {...}, "plugin": {"uuid": "my-plugin-name", "version": "1.1.3"}, ...}'
 ```
 
+## Custom Event Listeners
+
+The SDK allows you to create custom event listeners and events by extending the `EventListener` and `EventBase` classes. This is useful when you need to monitor data from external applications and perform specific actions in response to changes or alerts.
+
+### Creating a Custom Event Listener
+
+To create a custom event listener:
+
+1. Create new event model that inherits from `EventBase`.
+2. Create a new class that inherits from `EventListener`.
+    a. Implement the required `listen` and `stop` methods. The `listen` method should yield results as a json string that matches the new event model.
+    b. List the new event classes in the `event_models` class variable of the new `EventListener` class.
+3. Configure your plugin in its `pyproject.toml` file to use your custom listener.
+
+```python
+# custom_listener.py
+from collections.abc import Generator
+from typing import ClassVar, Literal
+
+from streamdeck.event_listener import EventListener
+from streamdeck.models.events import EventBase
+
+
+class MyCustomEvent(EventBase):
+    event: Literal["somethingHappened"]
+    ... # Define additional data attributes here
+
+class MyCustomEventListener(EventListener):
+    def listen(self) -> Generator[str | bytes, None, None]:
+        ...
+        # Listen/poll for something here in a loop, and yield the result.
+        # This will be ran in a background thread.
+        # Ex:
+        # while self._running is True:
+        #     result = module.check_status()
+        #     if result is not None:
+        #         yield json.dumps({"event": "somethingHappend", "result": result})
+        #     time.sleep(1)
+
+    def stop(self) -> None:
+        ...
+        # Stop the loop or blocking call in the listen method.
+        # Ex:
+        # self._running = False
+```
+
+### Configuring Your Custom Listener
+
+To use your custom event listener, add it to your `pyproject.toml` file:
+
+```toml
+[tools.streamdeck]
+    action_scripts = [
+        "main.py",
+    ]
+    event_listener_modules = [
+        "myplugin.custom_listener",
+    ]
+```
+
+The `event_listeners` list should contain strings in module format for each module you want to use.
+
+
 ## Creating and Packaging Plugins
 
 To create a new plugin with all of the necessary files to start from and package it for use on your Stream Deck, use [the Python SDK CLI tool](https://github.com/strohganoff/python-streamdeck-plugin-sdk-cli).