Add a plugin tutorial

tutorial/extensions.md

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+# Plugins
+
+JupyterLab can be extended in two ways: via top level application extensions
+or document widget extensions.  Application extensions extend the 
+functionality of JupyterLab itself and are the focus of this tutorial. 
+Document widget extensions extend the functionality of document widgets added 
+to the application and are covered in the Documents tutorial.
+
+The JupyterLab Application object is passed to all extensions and is 
+used to add content to the main area and the sidebars, as well as 
+add commands, keyboard shortcuts, and command palette entries.
+
+## Extensions(https://github.com/phosphorjs/phosphide/blob/master/src/core/extensionregistry.ts#L19) 
+Add content to the [application](https://github.com/phosphorjs/phosphide/blob/master/src/core/application.ts) itself or to services.
+- An example extension is the [editor widget](https://github.com/jupyter/jupyterlab/blob/master/src/editorwidget/plugin.ts), which registers an 
+editor widget factory with the Document Registry.
+- Extensions are activated explicitly through the Application [instance](https://github.com/phosphorjs/phosphide/blob/master/src/core/application.ts#L71). 
+- The extensions provided to app on startup, are activated automatically
+
+## [Services](https://github.com/phosphorjs/phosphide/blob/master/src/core/serviceregistry.ts) 
+Provide functionality to other parts of the application.
+- An example service is the [docucment registry](https://github.com/jupyter/jupyterlab/blob/master/src/docregistry/plugin.ts), which is used by extensions
+to add functionality around widgets backed by files.
+- Services are activated on-demand.
+
+- A full example is contained [here](https://github.com/jupyter/jupyterlab/tree/master/examples/lab).