Add usage example to README

Author: tpietzsch

README.md

@@ -1 +1,38 @@
-# scijava-listeners
+# scijava-listeners
+
+Helper class for maintaining lists of listeners.
+
+Usage example:
+```java
+public interface MyListener
+{
+    void somethingChanged();
+}
+
+public class Listenable
+{
+    // Create a variant of Listeners.List
+    private final Listeners.List< MyListener > listeners = new Listeners.SynchronizedList<>();
+
+    // Use Listeners.List.list to call registered listeners
+    private void notifyListeners() {
+        listeners.list.forEach( MyListener::somethingChanged );
+    }
+
+    // expose only Listeners (not Listeners.List) to allow un/registering listeners
+    public Listeners< MyListener > myListeners() {
+        return listeners;
+    }
+}
+
+public class Listening
+{
+    public Listening( Listenable l ) {
+        l.myListeners().add( this::notifyMe );
+    }
+
+    void notifyMe() {
+        System.out.println( "something changed!");
+    }
+}
+```