Add README

README.md

@@ -1,2 +1,42 @@
-# qupath-extension-py4j
-Experimental extension connection QuPath and Python via Py4J
+# QuPath Py4J extension 🐍
+
+This extension aims to provide a link between QuPath and Python using [Py4J](https://www.py4j.org/)
+
+## Installation
+
+You can install this extension the same as other QuPath extensions,
+as described in [the docs](https://qupath.readthedocs.io/en/latest/docs/intro/extensions.html#installing-extensions).
+
+Usually this involves using the built-in extension manager, or by downloading
+a build of the extension from [the releases](https://github.com/qupath/qupath-extension-py4j/releases)
+and dragging this onto a QuPath window.
+
+## Usage
+
+To start a Py4J gateway from QuPath, you can either click the Python icon on
+the QuPath toolbar, or use the menu item
+`Extension -> Py4J -> Start Py4J Gateway`.
+
+This dialog allows you to specify the port used for the websocket connection,
+and an optional authentication token:
+
+![A dialog box for the QuPath Py4J extension. A brief description of the purpose of a gateway, followed by "Port" and "Token" text fields followed by "Random" and "Copy" buttons, with "Cancel" and "OK" buttons at the bottom of the dialog.](gateway-options.png)
+
+Once a gateway is started, you can connect from a Python process using
+[qubalab](https://github.com/qupath/qubalab/):
+
+```python
+from qubalab.qupath import qupath_gateway
+gateway = qupath_gateway.create_gateway(port=25333)
+
+gateway.getExtensionVersion() # query the verison of the QuPath Py4J extension
+```
+
+Further documentation can be found in the [qubalab project](https://github.com/qupath/qubalab/).
+
+## Development
+
+It's recommended to use the extension setup described in
+[the QuPath extension template](https://github.com/qupath/qupath-extension-template?tab=readme-ov-file#set-up-in-an-ide-optional)
+for this extension.
+