Added custom plugin guidance

example_plugins/README.md

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+# Custom plugins
+
+Custom plugins allow users to create their own behaviors when a Meshtastic or Matrix message is detected. A sample plugin `hello_world.py` is provided.
+
+NOTE: Custom plugins are not supported with the Windows installer option.
+
+## Development
+
+Custom plugins should be written as a subclass of `plugins.base_plugin.BasePlugin` and given a file extension `.py`. The class should be given a unique `plugin_name`.
+
+## Installation
+
+Custom plugins should be copied to `custom_plugins` directory. They are detected upon startup of the relay.
+
+## Troubleshooting
+
+Each plugin has access to a `self.logger` that can be useful in troubleshooting runtime issues.