Plugins documentation / getting started guide (#835)

src/plugins/navigation/README.md

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+# Navigation
+As you can see this part is separate from the impress.js core code.
+It's because these navigation actions only need what impress.js provides with
+its simple API.
+This plugin is what we call an _init plugin_. It's a simple kind of
+impress.js plugin. When loaded, it starts listening to the `impress:init`
+event. That event listener initializes the plugin functionality - in this
+case we listen to some keypress and mouse events. The only dependencies on
+core impress.js functionality is the `impress:init` method, as well as using
+the public api `next(), prev(),` etc when keys are pressed.
+Copyright 2011-2012 Bartek Szopka (@bartaz)
+Released under the MIT license.
+
+***Author:***
+
+Bartek Szopka