Add example of a plugin capturing the impress:init event to plugin README

src/plugins/README.md

@@ -288,6 +288,41 @@ An init plugin is the simplest kind of plugin. It simply listens for the
 the plugin can initialize itself and start doing whatever it does, for example 
 by calling methods in the public api returned by `impress()`.
 
+The `impress:init` event has the `div#impress` element as its `target` attribute,
+whereas `event.detail.api` contains the same object that is returned by calling
+`impress()`. It is customary to store the api object sent by the event rather than
+calling `impress()` from the global namespace.
+
+Example:
+
+    /**
+     * Plugin A - An example plugin
+     *
+     * Description...
+     *
+     * Copyright 2016 Firstname Lastname, email or github handle
+     * Released under the MIT license.
+     */
+    (function ( document, window ) {
+        var root;
+        var api;
+        var lib;
+
+        document.addEventListener( "impress:init", function( event ) {
+            root = event.target;
+            api = event.detail.api;
+            lib = api.lib;
+
+            // Element attributes starting with "data-", become available under
+            // element.dataset. In addition hyphenized words become camelCased.
+            var data = root.dataset;
+            // Get value of `<div id="impress" data-plugina-foo="...">`
+            var foo = data.pluginaFoo;
+            // ...
+        }
+    })(document, window);
+
+
 Both [Navigation](navigation/navigation.js) and [Autoplay](autoplay/autoplay.js)
 are init plugins.