add events docs to runal

Author: Xavier Godart

content/runal.md

@@ -467,4 +467,23 @@ tail -f console.log
 
 The **console.log** file is deleted upon exit.
 
+### Events
+
+#### onKey(c, key)
+Listen to keyboard events. Key contains the key string. **It's a root function, which means it should be placed at the same level as **setup()** and **draw()**.
+```js
+// mySketch.js
+setup(c) { ... }
+
+draw(c) { ... }
+
+onKey(c, key) {
+  // save the current canvas in a png file
+  // when the "c" key is pressed.
+  if (key == "c") {
+    saveCanvasToPNG("canvas.png");
+  }
+}
+```
+
 </section>