Add example to listener documentation

listener.md

@@ -14,3 +14,26 @@ listener.listen([prop, ...], callback)
 listener.listen(prop)
 // Removes all callbacks from a given property
 ```
+
+## Example
+
+```js
+import Listener from 'listener.js'
+const listener = Listener({})
+
+// Listen for any changed property
+listener.listen("*", (value, prop) => console.log(`${prop} changed to ${value}`))
+
+// Listen only for changes to the foo property
+listener.listen("foo", prop => console.log("it was foo, by the way"))
+
+// Several listeners for one property are possible
+// They will be executed in order of definition
+listener.listen("foo", prop => do_something())
+
+listener.foo = "New Value"
+// Triggers 3 handlers
+
+listener.bar = "New Value"
+// Triggers only the * handler
+```