docs: add Protégé graph plugin build and install instructions to README

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: lroffia

README.md

@@ -60,6 +60,35 @@ Expose that WebVOWL service at a public URL, for example `https://ontologies.exa
 
 The ontology files must also be reachable by the WebVOWL service. Local-only URLs such as `localhost` cannot be fetched by a remote WebVOWL service.
 
+## Protégé Graph Plugin
+
+A Protégé Desktop tab plugin that renders a force-directed graph of the active OWL ontology. Classes are shown as rounded boxes containing datatype properties, connected by curved edges for `rdfs:subClassOf` and object property relations with labels at the arc midpoint.
+
+### Build
+
+Requires Maven and JDK 8+.
+
+```sh
+cd protege-graph-plugin
+mvn clean package
+```
+
+The plugin JAR is generated under `protege-graph-plugin/target/`.
+
+### Install
+
+Copy the generated JAR into the `plugins/` directory of your Protégé Desktop installation, then restart Protégé. Enable the tab from **Window > Tabs > VAIMEE Graph**.
+
+### Features
+
+- Force-directed layout with D3-style simulation (charge, link distance, gravity).
+- Interactive drag: grab a class node and connected nodes follow via spring forces.
+- Pan (drag background), zoom (mouse wheel), and fit-to-view on reset.
+- Namespace legend with color-coded checkboxes to show/hide classes.
+- Layout settings panel with sliders for charge, link distance, link strength, and gravity.
+- `rdfs:comment` annotations displayed inside class nodes with word wrapping.
+- Curved edge routing around intermediate nodes.
+
 ## Build Locally
 
 Build the nginx image: