Add lab extension documentation

tutorial/index.rst

@@ -20,6 +20,7 @@ Contents:
 
    repo.md
    plugins.md
+   labextensions.md
    documents.md
    notebook.md
    patterns.md

tutorial/labextensions.md

@@ -0,0 +1,30 @@
+# JupyterLab Extensions
+
+A JupyterLab extension provides additional, optional functionality to JupyterLab's built-in capabilities. An extension is a module that provides one or more [plugins](./plugins) to the JupyterLab application. To streamline third-party development of extensions, this library provides a build script for generating third party extension JavaScript bundles.
+
+For information on creating a lab extension, please see the documentation for
+the [jupyterlab-extension-builder](https://github.com/jupyter/jupyterlab-extension-builder).
+
+
+## Installing and enabling extensions
+
+You can install your labextension with the command:
+
+`jupyter labextension install path/to/my_extension my_extension [--user|--sys-prefix]`
+
+The default installation is system-wide. You can use ``--user`` to do a per-user installation,
+or ``--sys-prefix`` to install to Python's prefix (e.g. in a virtual or conda environment).
+Where my_extension is the directory containing the JavaScript files.
+This will copy it to a Jupyter data directory (the exact location is platform
+dependent - see :ref:`jupyter_path`).
+
+For development, you can use the ``--symlink`` flag to symlink your extension
+rather than copying it, so there's no need to reinstall after changes.
+
+To use your extension, you'll also need to **enable** it, which tells the
+notebook interface to load it. You can do that with another command:
+
+`jupyter labextension enable my_extension [--sys-prefix]`
+
+There is a corresponding ``disable`` command to stop using an
+extension without uninstalling it.