docs: Add example for a simple script plugin

docs/plugins.md

@@ -17,15 +17,41 @@ These entry points are known by `gallia`:
 : List of callables which get called during the initialization phase of the `ArgumentParser`; can be used to add new categories to the CLI.
 
 `gallia_transports`
-: List of subclasses of {class}`gallia.transports.BaseTransport` add a new URI scheme for the `--target` flag.
+: List of subclasses of {class}`gallia.transports.base.BaseTransport` add a new URI scheme for the `--target` flag.
 
 `gallia_uds_ecus`
-: List of subclasses of {class}`gallia.services.uds.ECU` which add new choices for the `--oem` flag.
+: List of subclasses of {class}`gallia.services.uds.ecu.ECU` which add new choices for the `--oem` flag.
 
-## CLI Categories
+## Example
 
-TODO
+Below is an example that adds a new command to the CLI (using {class}`gallia.command.Script`).
+Let's assume the following code snippet lives in the python module `hello.py` within the `hello_gallia` package.
 
-## Commands
+``` python
+from argparse import Namespace
 
-TODO
+from gallia.command import Script
+
+
+class HelloWorld(Script):
+    """A hello world script showing gallia's plugin API."""
+
+    COMMAND = "hello"
+    SHORT_HELP = "say hello to the world"
+
+
+    def main(self, args: Namespace) -> None:
+        print("Hello World")
+
+
+commands = [HelloWorld]
+```
+
+In `pyproject.toml` using `poetry` the following entry_point needs to be specified:
+
+``` toml
+[tool.poetry.plugins."gallia_cli_commands"]
+"hello_world_commands" = "hello_gallia.hello:commands"
+```
+
+After issueing `poetry install`, the script can be called with `gallia script hello`.